It doesn't make sense to try and support UDF writing inside CDBurn - UDF needs a real filingsystem to be a sensible thing. If you keep withing the "generate image, write image" kind of working of CDBurn, you would only have disadvantages using UDF.
So it would be a seperate product. However, UDF is a very complicated thing, so unless someone ports the code from another OS, I don't see this being likely to happen. If someone wants to have I shot, I would be happy to provide some insight in how to handle write operations on CD/DVD writers on RISC OS.