I don't know. Is this option available for 'cosmic vending machine' rituals? (ie. can my Pure Mortal walk around with a Walker in his back pocket?)
This is about the only way I can think to allow it to work. It would essentially be an extremely overpowered enchanted Item, but it would be heavily limited in both frequency of use and time of implementation. So it wont be available every time you might want to use it (based on some predefined schedule), and it also will stop working (or get progressively less powerful) with each use; both would be up to the DM to adjudicate.
It's the sort of thing that I might consider as a way to shore up an otherwise unbalanced character that is having a hard time surviving with the rest of the party (say a purely social character in a party full of bruisers). Otherwise it's mostly just sidestepping the system's balance. Philosophically I always think that such things can always be worked around for the fun of all, but in practical application you are loosing some of the benefit of a tested and balanced system, and it will take an awful lot of extra effort and attention on the DM's part to keep it from slipping away.
Another key point is also the actual nature of the spell. A raw damage spell is clearly going to upset the balances, if one character is walking around with a bazooka in their pocket while the rest have pea shooters. On the other hand, a ridiculously overpowered teleportation spell that is juiced enough to carry the whole party and bystanders makes the sort of plot device that might not actually impact too much once the DM knew to account for it. So stay away from direct offensive magics or anything that would be directly opposed, but things like transportation, inter-party communication, or anything that is more social can work (so long as the DM realizes that it become Off the Table as a challenge/puzzle).