I don't quite see what's wrong with just making a dedicated equipment slot... the equipment itself doesn't have to do anything in the game, that could be done with something like a script that's common to each and every mission in the mod/campaign.
Making this work in multi could be even more difficult than bumping the bank limit. You're adding a new feature, as opposed to expanding an old one. This feature would also need new interface, HUD, pilot code (for carrying equipment through Red Alert missions), etc.
Antipodes 8 has got the pilot code covered. Dedicated code for a HUD interface is unneeded for the most part, as you can use custom gauges + scripting to achieve any effects you need to display, although for some things such as mid-mission equipment changes will need some tweaks to the HUD and scripting code.
A new Interface page will definitely needed, and should be configurable. There's been some work on modularizing the menu/interface system already, so hopefully we'll see some news of that soon.
Multiplayer is a topic in of itself, I don't know if what they have currently verifies all files used client side match the files server side... which is a common (?) way of determining if somebody's trying to cheat, or if they're having a derp moment and forgot to take mod-stuff out of the folders while playing multi
).
Unfortunately, checksums alone cannot tell which modifications are merely cosmetic (which may be allowable) or alter the performance of in-game assets (which should be illegal), unless it's known specific files/tables affect only cosmetic stuff.
