Thanks for the feature suggestion, @joneppie - but we already have something that lets you do this in one of two ways. Use Flux's hooks. There is one for when the form is built (which receives the TCEForms DS array) and another for when the form is retrieved from a template (which receives the PHP form object instance).

See FluidTYPO3/flux@1f76e8a for further information how to subscribe to hooks. The HookHandler class has constants with names of all hooks that you can subscribe to and if you "find all usages" in your IDE you will see where exactly in the code each one is called and what variables are in the payload data.