I don't really agree with restriction-based loot systems. However, if you do think that priorities are necessary, here's a system I came up with a long time ago (but never actually posted about):
Have your class officers draft the items in the instance.
In a random order, each class picks one item. That class gets first priority on the item. Continue drafting items (use switchback so that the second pick goes in the reverse order). The second class to pick an item gets second priority on that item. Continue until all the non-class restricted items have a first and second priority.
Each class now has an equal number of priorities. The focused classes will probably get first priority on the very best stuff, but hybrids should be able to score some decent second priorities.
As well, it sets up little rivalries between classes who do not normally compete, such as mages and warlocks, which helps them understand what it feels like to be a hybrid.
Plus it's a fun game, and you could turn it into a guild event where everyone provides "advice" to their officer.