http://www.iana.org/assignments/mail-parameters, but as far as I can tell, there's no registry of trace headers.There is no registry of trace headers because there are, and have been since RFC 821, only two of them: "Received:" and "Return-path:".
If you believe what the RFCs say, there are many more trace headers than those two. RFC 5322 makes a special case of Resent-foo: fields, but they act an awful lot like trace headers. RFC 5451 says that Authentication-Results "should be treated as a Trace Field." RFC 5436 says "The 'Auto-Submitted' header field is considered a 'trace field'". RFC 4408 says that "The Received-SPF header field is a trace field." RFC 5518 says "VBR-Info is a 'trace header field'". That horse left the barn a long time ago. RFC 5537 also lists a bunch of trace fields for usenet, which isn't mail, but it might be a good idea to reserve them to avoid collisions.
I'm suggesting that the time may have come to add one or more rather than continuing to overload "Received:".
It's certainly time for a trace field registry. And I suppose that if we have one, adding new trace fields isn't that big a deal. R's, John