I haven't tried to put lipglass in the lipmix palette, but I dont think it would work very well since the case doesnt close airtight, like a lipglass tube would, so it would probably dry out quickly. The lipglass wouldnt harden like a lipstick does, since it doesnt have as high a wax content and it might just harden to a dry clump. Before it competely dries out, it would probably still be runny and if you turned it on its side at all it might make a mess.
I think to make a lipglass palette, you might want to try one of those pill a day containers where each slot opens individually and snaps shut, rather than MAC's lipmix palette.