The unique object generator should be looking at the group and randomly picking a realm to use, so some items should be usable. I'll review that section of code and see if it can be improved.
As for armor, everything but helmets / caps
can be cross realm, but head gear is all realm specific. We'll probably need to add some special checks to get cross realm gear working. On live you can also substitute certain skills for others. Sword, Slashing, and Blades will all give a bonus to each of those skills, for example. We aren't close to being able to support that yet.

Perhaps we can create an item tansmorgiphying npc that will convert items meant for one realm into another?
There are also weapon model problems with certain races that make the weapon look goofy when held. In short it's a difficult problem.
My other concern s far as armor goes is quick identification in RvR. Mids look like mids, etc. due to the different armor styles. Hib cloth is different from mid cloth which is different from alb cloth. So even though you could wear it, and except for helms it would look correct, we might not want to enable this. Unique armor styles is something new to the unique object generator to make it more like live.