I guess in theory it may be possible but that is based on the simple rule of thumb that the host and scion must be compatible. Considering that a satsuki is from the Genus rhododendron and of course a rhododendron is also from the Genus rhododendron it meets that requirement. I do not know if it matters greatly that they are each from separate subgenus.
According to the The Grafter's Handbook by RJ Garner "the botanist classification only serves as a rough guide to compatibility, for it is founded upon the reproductive characters, and experienced grafters have learnt that this is not a reliable guide." Garner goes on to discuss in great detail how permanent unions have been established across one Genus to another, however it is uncommon. Incompatible unions are the distinct failure to unite of a mechanically sound union.
Great concept. If you know how to graft why don't you give it a try? There is plenty of information out there on how to graft and for the most part the tools and supplies are inexpensive. If I were to attempt this I believe I would attempt a thread graft. This seems to be the typical way that branches are added on an azalea and it would provide cambium contact while the donor plant sustained the scion. After a season or two the branch could be severed from the donor plant if the exit looks healed.
The Underground Gardens in Fresno has a citrus tree planted below ground with a skylight leading to the surface. This citrus tree has 7 different fruits grafted to it. Some are very different like grapefruits and tangerines on the same tree. The grapefruit is called a Chidero (sp.). This historic museum is only about two miles from my home. A very coool place during summer.
I would go for it. I would wonder though if it would be worth it, since Satsuki seem to fatten up fairly quickly anyway. Common Rhodo do not have as attractive a trunk as Satsuki do. They look more shrubby like Oleander.
I agree, even if it's possible, it seems a bit pointless as satsuki bulk up quite well and produce better looking trunks ultimately. It would be a nice thing to try if you have found a particularly good looking rhodo trunk, but other than that...?
Rhodies are also much hardier...I'd love to have a satsuki but I doubt it would survive my winters, even mulched in my garage . It would be interesting to see if rhody/satsuki graft would provide more winter hardiness. Good luck, Bill
Rhodedendron impeditums (chinese sub-alpine shrub) seem to be quite abundant and thus affordable on the european market right now. Very hardy too. Some of these have tiny leaves (~10mm under nursery conditions). The one I have seen flower most profusely under nursery conditions is 'Album' small white flowers. The bark on these gets interesting very fast also. Maybe a graft candidate.