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While designing my MMC3 cart I thought of using the pins of the expansion port (16 ~ 20 and 51 ~ 55) for the CPLD JTAG port and the #WE of the flash chips. But I don't know if these pins are used by any peripherals that might make my cart go nuts.

Any info about this?

The only official use of the EXP pins is as documented on the wiki, and this (underdocumented, unused) test cart.

The various in-place flashable carts I've seen have been making use of the EXP pins for programming, but I don't know if there's any documented conventions.

Note that INL's carts add a separate card-edge connector for JTAG for the CPLD.

Thanks for the fast and complete answer! It looks like as long as I do not use EXP 0, 2, 5, 6 and 9, there should be no problem.

As long as the expansion devices aren't permanently connected, I would expect that you could probably use EXP 5 as well if you need another pin (in case five extra pins isn't enough), since it may be consider as a cartridge-specific features, so you could use it for your own purposes as well. (However, I am not quite sure; if other people understand then it could be answered better?)

Nice suggestion, thanks!

My GTROM board uses EXP4 for programming the lockout chip, but it's done through an extra connector which is basically a jumper and is only installed during programming.

This is not widely used, but here are the EXP pin definitions I've used on the 8TROM board. Probably pretty close to what I'll use in the future. It was to be compatible with the ENIO CPU board.