I don't really have a solution for your issue, but to be honest...I do recommend moving your caches to HD. Caching files becomes kinda pointless when you lose all the cache contents everytime you shut down or reboot your PC (that is, unless you save an image of the RAM disk to HD).
It simply isn't worth it, as there won't ever be so many cache reads/writes that they would be noticeably slower using the HD than they would in RAM.
As for thumbnails, definitely keep them on, since otherwise all the image files will be read and read over and over again to create the thumbnails. But the thumbs.db files themselves are pretty small and not worth obsessing over - why not just leave them on the SSD?
Truth is, you'll never completely stop Windows from writing all kinds of temp files and logs to the system partition, even in folders you wouldn't suspect of holding such files. I haven't even mentioned things like spyware/antivirus software and their signature databases yet, or Windows updates which are downloaded to C:\Windows\SoftwareDistribution\Download and then installed to other locations...
Again, thumbs.db files are relatively tiny (a few 100s of KB at most) so I just wouldn't worry about them.