I've never bottom painted, but can tell you how one of the local fiberglass repair men says he does it on bunk trailers, and I suppose it would work on rollers, as well.
His trick is to jack up the front of the trailer as far as it will go with the trailer's own jack, block the keel toward the bow, and lower the trailer jack while backing off on the trailer's winch. The hull ends up being supported in three places: the keel, and the two aft bunks.
Seems workable to me, so long as the keel block is secure. I'd block the trailer tires, and also be inclined to put jack stands under the axle, so that a flat tire doesn't cause a bad day. I have messed with this approach, just to see if it works, and just enough to take all pressure off the front bunks. This would allow me to remove them, one at a time, and do whatever bottom work was necessary. You'd want to make darn sure your keel block couldn't slip, to avoid punching a hole in the bottom of the boat. I made up a slotted frame which had a channel about half the depth of the keel, running along the keel, and padding on the sides so it couldn't tip.
I haven't messed with the stern, but it would work pretty much in reverse of the bow: lower the trailer's jack, block the keel at the stern (or, better yet, the transom), then raise the jack. I'd be inclined to raise the front of the trailer with a floor jack when doing the stern, since this would be a good way to overload the trailer jack. Jack stand the front of the trailer, and you should be set to go.
Dont' know if this works, but it's how I'd head into it. Let us know what you do.