I'm sorry that I don't know typescript yet to more deeply contribute to the project. In an effort to help with the groundwork to capture the XHR calls I've written the following gist:

https://gist.github.com/ucjonathan/9adc68ee74b331f6da5e0a13e1652dac

This is a vanilla JS piece of code that will collect the XHR request/response details into an object and then trigger an event that can be listened to. I've mode provisions in the code to:

1. Allow for a custom function to perform redaction on the log record before further processing (masking of sensitive data that could be in the request/response.
2. Allow for a custom function to suppress a particular capture. You don't really want to capture the XHR call you're making to store the events that RRWeb is collecting on your server. That would create a nasty feedback loop.

I'm hopeful that this code could be used as the foundation to start adding that XHR capture support as a new event type similar to what was done with the console logging.