What a head f***. Why wouldn't you just have a whole seperate rig for guitar?
Sorry, that's not very helpful. Seeing as you're taking up a channel for the guitar mic, and then a channel for bass DI, maybe asking for an extra mic for bass would be pushing it, in which case a DI with some sort of amp sim that resembles the colouration of your bass rig is what you need.
Having said that, it sounds like your band is so big that one extra mic wouldn't hurt!