I presume an external tap will no longer work with such a mod, seeing as you'd be daisy chaining two taps together. Removing the ability to play five player Bomberman, Dungeon Explorer, etc. pretty much ruins the system for me.
Unsure about the daisy chain (has anyone tested to see if 2 of them work? i imagine you would just use the last tap in the chain and still only be able to use 5 players tho..). I don't have a need for 5 player bomberman or 5player anything. Just me and wife play on this and i'm just fine with that. If that need ever arose, i'll just use my unmodded TG16.
The way it seems to work is that a clock comes in and it switches ports every clock edge. So by daisy chaining them, my current thinking is that you'd only have access to the first port of the downstream multitap, and then only once out of every 5 samples (so every button press would be like an auto-fire). However, there is something about the multitap detecting the DIN connector shells, and skipping the unconnected controllers, but I kind of doubt that to be true, because there's no way to convey which controller's data is being sent up, generally (it resets, then polls all 5, as I understand it).
In short, it seems highly likely that there will be no way to chain an extra multitap to this. (Maybe it'd make sense to wire up the other three to a DB25 connector somewhere, and make a breakout cable or something, for those times when Bomberman must be played?)