Given you said you "made a complete mess of the rear" you may not have a choice

If the rim is FAR out of true the only reasonable way to fix it is to detension the spokes and retrue. If you have a center reference point on the stand dishing the wheel is really no different than lateral truing...you're just truing to a center point rather than against the rim itself, if that makes sense. To try to minimize the amount of effect on the dish detension slowly, half turn per spoke and keep going around the wheel until it starts to loosen. If the dish ends up being far off I don't kow that you can do it right without loosening the wheel all the way. I'm afraid that with a little ternsion left in it you'd end up with too much tension before the dish came back for you.

Also, we're not talking about a bent rim at all here are we?