Ok, just to throw a monkey-wrench in here, what about Beast Companions that are Also Mounts or Mounts that are Also Familiars? From a few Dragons back, we got the Beast Ranger who rides his Horse Companion (as well as 1 of the Martial Powers)...and ages before that we got the Ritual that lets you enlarge your Familiar as well as give it the Mount keyword. (I think the Rules dictate easily the Mount/Companion clash, but the Familiar/Mount class? Not so sure)
Could you double stuff these items (Familiar Item Slot, Mount Item Slot, etc) if your Steed was a little weird/split in their identity?
-Jared
"My DM said No for the sake of Sanity (Which I agree with), but I figured since someone brought the topic up, I'd throw the madness out there."
PS: While it's a tangent, I like the arguments concerning the Essential Paladin's Mount Summoning & whether it's the "same" mount every time. The point being if it is, you can equip the thing like you would a normal mount. Which it isn't. But is. Kinda. But Not. Still, fun thought experiment.