From a technical standpoint, custom guild houses wouldn't really work. If someone manages to sneak a npc into their guild house, and the admin reviewing it misses it, you could basically cause a huge issue with the server, limited only to the capabilities of gscript2.

You can give yourself millions of gralats, dc people, warp people, and do all tons of stuff just if an admin misses 1 npc reviewing one level. maybe if they used something like an online tile editor it might work, but that would be very hard to make.