We use them regularly for one off requests of full backups.- we don't even bother to take them out of the original policy, but that's our choice and safer if you should forget to add it back!
We keep them there & just amend the client, calendar, backup selection or whatever.
The only way I can think of at the mo utilising your current policy would probably be a right royal pain to administer - create your new FULL schedule & then add exclude lists for that specific schedule for everything/all drives or whatever on all clients except the one you want!!
Much much easier to create the new policy, plus it won't interfere with your current incrementals
