diff --git a/CHANGELOG.md b/CHANGELOG.md
index 68c67514d77c..e00cc09614e0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,6 +15,7 @@ Changes:
 - Add runtime `ParachainHost` version 5
 - Consistently use `import.meta.url` inside tests
 - Group `Uint/Int` tests for maintainability
+- Serialize any additional payload fields via `toJSON()`
 - Update to latest Polkadot, Kusama & Substrate metadata
 
 
diff --git a/packages/types/src/extrinsic/SignerPayload.spec.ts b/packages/types/src/extrinsic/SignerPayload.spec.ts
index 0f64694ff4b8..55d681b19816 100644
--- a/packages/types/src/extrinsic/SignerPayload.spec.ts
+++ b/packages/types/src/extrinsic/SignerPayload.spec.ts
@@ -60,6 +60,29 @@ describe('SignerPayload', (): void => {
     });
   });
 
+  it('handles Option<AssetId> correctly', (): void => {
+    const test = new SignerPayload(registry, { assetId: 123 });
+
+    expect(
+      [...test.keys()].includes('assetId')
+    ).toEqual(true);
+
+    expect(
+      // @ts-expect-error We don't have getters for this field
+      test.toPayload().assetId
+    ).toEqual(123);
+
+    expect(
+      // @ts-expect-error We don't have getters for this field
+      new SignerPayload(registry, { assetId: 0 }).toPayload().assetId
+    ).toEqual(0);
+
+    expect(
+      // @ts-expect-error We don't have getters for this field
+      new SignerPayload(registry, TEST).toPayload().assetId
+    ).toBeUndefined();
+  });
+
   it('re-constructs from JSON', (): void => {
     expect(
       new SignerPayload(registry, {
@@ -82,7 +105,7 @@ describe('SignerPayload', (): void => {
   });
 
   it('can be used as a feed to ExtrinsicPayload', (): void => {
-    const signer = new SignerPayload(registry, TEST).toPayload();
+    const signer = new SignerPayload(registry, { ...TEST, assetId: 123 }).toPayload();
     const payload = registry.createType('ExtrinsicPayload', signer, { version: signer.version });
 
     expect(payload.era.toHex()).toEqual(TEST.era);
@@ -90,5 +113,7 @@ describe('SignerPayload', (): void => {
     expect(payload.blockHash.toHex()).toEqual(TEST.blockHash);
     expect(payload.nonce.eq(TEST.nonce)).toBe(true);
     expect(payload.tip.eq(TEST.tip)).toBe(true);
+    // @ts-expect-error assetId is of unknown type, so we don't know about "isSome"
+    expect(payload.inner?.get('assetId')?.isSome && payload.inner?.get('assetId')?.eq(123)).toBe(true);
   });
 });
diff --git a/packages/types/src/extrinsic/SignerPayload.ts b/packages/types/src/extrinsic/SignerPayload.ts
index d81ac31d522f..f2ec372034ab 100644
--- a/packages/types/src/extrinsic/SignerPayload.ts
+++ b/packages/types/src/extrinsic/SignerPayload.ts
@@ -2,7 +2,7 @@
 // SPDX-License-Identifier: Apache-2.0
 
 import type { Text, Vec } from '@polkadot/types-codec';
-import type { Registry } from '@polkadot/types-codec/types';
+import type { AnyJson, Registry } from '@polkadot/types-codec/types';
 import type { HexString } from '@polkadot/util/types';
 import type { Address, BlockHash, Call, ExtrinsicEra, Hash } from '../interfaces/index.js';
 import type { Codec, ICompact, INumber, IRuntimeVersion, ISignerPayload, SignerPayloadJSON, SignerPayloadRaw } from '../types/index.js';
@@ -112,18 +112,22 @@ export class GenericSignerPayload extends Struct implements ISignerPayload, Sign
    * @description Creates an representation of the structure as an ISignerPayload JSON
    */
   public toPayload (): SignerPayloadJSON {
-    const result: Record<string, string> = {};
+    const result: Record<string, AnyJson> = {};
     const keys = Object.keys(this.#extraTypes);
 
     // add any explicit overrides we may have
     for (let i = 0, count = keys.length; i < count; i++) {
       const key = keys[i];
       const value = this.getT(key);
-      const isOption = value instanceof Option;
 
       // Don't include Option.isNone
-      if (!isOption || value.isSome) {
-        result[key] = value.toHex();
+      if (!(value instanceof Option) || value.isSome) {
+        // NOTE In the spread below we convert (mostly) to Hex to align
+        // with the typings. In the case of "unknown" fields, we use the
+        // primitive toJSON conversion (which is serializable). Technically
+        // we can include isNone in here as well ("null" is allowed), however
+        // for empty fields we just skip it completely (historical compat)
+        result[key] = value.toJSON();
       }
     }
 
diff --git a/packages/types/src/types/extrinsic.ts b/packages/types/src/types/extrinsic.ts
index 27a77dc17ecd..c934f174364f 100644
--- a/packages/types/src/types/extrinsic.ts
+++ b/packages/types/src/types/extrinsic.ts
@@ -38,22 +38,22 @@ export interface SignerPayloadJSON {
   /**
    * @description The checkpoint hash of the block, in hex
    */
-  blockHash: string;
+  blockHash: HexString;
 
   /**
    * @description The checkpoint block number, in hex
    */
-  blockNumber: string;
+  blockNumber: HexString;
 
   /**
    * @description The era for this transaction, in hex
    */
-  era: string;
+  era: HexString;
 
   /**
    * @description The genesis hash of the chain, in hex
    */
-  genesisHash: string;
+  genesisHash: HexString;
 
   /**
    * @description The encoded method (with arguments) in hex
@@ -63,22 +63,22 @@ export interface SignerPayloadJSON {
   /**
    * @description The nonce for this transaction, in hex
    */
-  nonce: string;
+  nonce: HexString;
 
   /**
    * @description The current spec version for the runtime
    */
-  specVersion: string;
+  specVersion: HexString;
 
   /**
    * @description The tip for this transaction, in hex
    */
-  tip: string;
+  tip: HexString;
 
   /**
    * @description The current transaction version for the runtime
    */
-  transactionVersion: string;
+  transactionVersion: HexString;
 
   /**
    * @description The applicable signed extensions for this runtime