I don't think a rare, one-use item that acts like a portable Megasphere would be unbalanced. Or even a portable invincibility power-up. Sure, it would make the Cyberdemon a cakewalk, but for every player who successfuly used it on Cybie there would probably be five who died with it in their inventory because they were saving it for later.
A jar that lets you collect powerups would definitely be unbalanced, though.
...All things considered a portable one-use invulneurability might be awesome. I think I'm all for it - it would definitely help out in a pinch, and yeah, invariably people would die with it in their inventory anyway, just like health kits.
And I know there's the part of "Easy access to Cormack", but it would be exotic or unique, so frankly your chances of running into it are about as likely as running into that perfectly timed invulneurability right now
Not sure how I feel about a portable Megasphere, but a portable Supercharge would probably be alright (A super medkit of some sort)
Also, the armor repair kit sounds great, but as it currently stands, it would make Ammochain builds even more ridiculous than they currently are, even if the ammo cost was increased.