In reply to:

it might be possible to manually craft a Xing header that would give the player application a roadmap to better locate positions in the file. After all, a CBR file is only a VBR file that doesn't vary very much...

That's a cunning move, Paul. You should be able to write a constant VBR map (as the values will be 0, 1, 2, 3, ..., 255 for all CBR files), and simply concatenate the audio file onto it.
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Toby Speight
030103016 (80GB Mk2a, blue)
030102806 (0GB Mk2a, blue)