Sending a header at the beginning of the UDP stream does not fit with the idea of UDP streaming. It'll only work if you're lucky, so I'd rather not implement it at all. I don't want to clog the code and spend extra time on it until I see a clear need. For now, besides artists, the potential need is low-latency monitoring. It can be done in MP3 or audio/basic for now, without headers; that should be enough for first tests.
Anyway, people will always need to specify a mime type for UDP inputs. This type can carry all the header information, as with audio/basic. When needed we can create conventions on more mime-types, for example audio/cd for stereo 44100Hz s16le (we can even come up or re-use a syntax for mime types describing all kinds of PCM streams). This should be a good plan for PCM. Concerning OGG I think there are much bigger problems than the header/mime-type, since the loss of one packet desynchronizes everything iirc.
Two more relevant improvements for UDP I/O in my opinion: (1) check why AAC+ doesn't work (while it does over TCP with icecast) and fix it; (2) possibly send metadata as out-of-band data. I'm not planning to touch those before I see a user's need, but I'll spend my coding time fixing release-critical bugs.