I've always wanted a small roth, and have thought many times about how to go about getting one, or getting *to* one through breeding. I heard once from Norito Hasegawa that he'd seen a compact roth, but when I asked him about it in more detail, it turns out he was referring to a roth that was simply a bit more compact than others.
Creating mini-roths would be fantastic, but unfortunately, would require a great deal of work from scratch. If you were going to stay completely within the species, you'd have to find roths that are small and compact to begin with. I can say that in the sib crosses I've seen, there are always a few smaller ones, but I would suspect that these smaller ones are the runts of the litter and will grow more slowly as well. Current breeding directions push bigger flowers, rather than smaller plants, since it is the flower which is the driver behind roth prices, so you would definitely be swimming against the tide by trying to get small plants out of the progeny of the many current roth crosses.
Per goldenrose's comment above -- I happen to have a plant of which 'Noyo' was the parent. It is compact, but very, very slow growing. I've had seedlings mature faster than this plant has put out a new leaf. Maybe it just doesn't like my conditions, I'm not sure...
If you were seeking to create a roth-like plant, then going with the wilhelminiae hybrid approach is a good one. I'm sure it could be done given enough generations and selection, but I suppose you'd have to have great amounts of patience and persistence in equal measure.