I’m fairly certain that if you want the benefits of the Platinum status you have to use the miles from the Platinum account. It’s the same with electronic elite upgrades.
But I wonder if AA will let you waitlist without sufficient miles in your account, and once they clear then you call, split the reservation, and use the individual accounts to pay for the upgrades. I feel like this is something that probably would have worked years ago but maybe it’s more automated now.

Of course the other caveat is doing it all on the same PNR will require 4 upgrade spaces available at one time before they will all clear, which they can only do concurrently. I’m not sure of the practical implications of that, I.e., I don’t think you’d get skipped over if there was only, say, one upgrade released.