Multi-select is like communism: it's a good idea, but it doesn't work. I mean, how many CRPGs have multi-select in inventory? Off the top of my head, I can't recall any (BG1 / 2 had multi-select in the shopping interface, but not in the inventory per se).
I suspect that contextual interpretation of a mouse click becomes fiendishly difficult, if not impossible. The game sees that you clicked an item, but what did you mean by that? Did you mean to pick it up? use / consume it? start multi-selection with more clicks to follow? start multi-selection by dragging a rectangle?..
I think the best approach here is to assign modifier keys for most common actions. We already have Shift+Click assigned to stack splitting (sort of); why not assign, say, Alt+Click to the "Send To Camp" action? I don't see why it won't work.
