I checked through all the docs I have and I came up with nothing. =( The only mention of the multitap is on page 9-2 of the SGL Library. It says that it's scheduled for release... An interesting thing is they mention a device called the "SEGA Tap" which connects 4 megadrive (genesis) controllers to the saturn. I never heard of such a device. Sorry for being of absolutely no help.
I think the only logical thing to try would be Smpc_Peripheral...but I'm sure you already tried that.
I'm putting the finishing touches on my first multitap game (pong clone, poor collision detection, no sprites, just slPrint statements, and one color, oh yeah it sucks) and I wanted to ask how if anyone knows how to program the multitap. Looking at Vreuzon's code, it seems that smpc_peripheral[0-5] are for the first multitap. Does that mean that 15-20 are for the second multitap? Anything else I should know about the multitap? Thanks in advance.