If you want to override all the doctypes, just make a list of the (around 500 doctypes in ERPNext + Frappe + some apps) and add them to doc_events in hooks.py. But I would advise against overriding all the doctypes and all the events.
Okay, maybe you can automatically generate a Server Script for your “Custom Webhook” document. In you “Custom Webhook” doctype, on save, generate a Server Script for the specific combination of doctype + event you want to inspect.
You don’t have a lot of choice I think, everything happens in Document.run_method and there is not a lot of overrides possible there.
If you know the list of (doctype, event) pairs at runtime, you can generate Server Scripts with a single line frappe.call("your.whitelisted.method", doc, "{{event}}"), but other than that I don’t know an answer to your original question.