Most bunnies do not like to hop/walk on slippery floors- although they generally do like to lay on tiles to cool down and rest. I would think a bathroom floor would be just fine, just make sure to give your bunny something to get off of the slippery floor- like cardboard, fleece blanket, etc. The advantage to the bathroom floor is that for littertraining it should be easy to clean as your bunny is first learning.
Look around the bathroom at your bunnies eye level to see if there is anything the little rascal might get into- caullking around the fixtures, cords, etc- and then remove or bunny proof any possible problem areas.
Are you planning on closing he door- or are you putting up a barrier to keep the bunny in the bathroom when the bun needs to be confined? A barrier or baby gate might help the bunny to still feel like a part of the household even if the door is closed. But you would have to make sure you use something secure and small enough that they can not slip thru or shove aside.
One of my bunnies- Cotton- now that she is about 1 1/2 years old almost no barrier will stop her unless it is permantly installed- like a door or baby gate- or firmly wedged in place by something so heavy she can not budge it. She is like a cat- if the area is blocked off- she wants to get in there BAD- and she will shove, bite, dig and try to toss the barrier until she gets her way.
EDIT: sounds like a beautiful bunny- you will have to post pics once you pick up your bunny!!!!