Very odd hack... Hmm.

I guess the obvious thing is you want the bitrate of the MP3 to be very low. LAME seems to only want to go as low as 32 kbps with MP3, but will go lower (down to 8) with MPEG-2 and MPEG-2.5 . Assuming your MP3CD player is happy with those formats, you might try one of them.

I wouldn't bother with WAV files. LAME can take raw PCM data as input, so I'd just create a file full of all zeroes (use dd on linux, or whatever on Windows) and see how LAME does with that. I don't know for sure that encoding complete silence is lossless (i.e. it won't introduce any artifacts) but if it did, I'd be surprised.

So, now to your question of how long the input file should be... Well, that's trial and error. To lessen the size of the file you'll have to create, you can probably give LAME a sampling frequency argument, -s, which I think only goes down to 8 Khz.

Experimenting with this a little, I have a 100MB file of all zeroes here, and encoding it with this command line, I get a 53 minute MP3 that's only about 12MB in size:

lame -b 32 -s 8 foo.pcm foo.mp3

So, I guess you could fit about 50 of those on a CD-R, so that's around 44 hours... Seems long enough...