It feels like I'm finally getting somewhere. My GIO migration branch of Thunar now generates thumbnails using the D-Bus service provided by Tumbler (which is still in development but almost ready for a release). It has no references to ThunarVFS anymore, except for one function required by Thunarx, the Thunar extension library. The next step will be to prepare thunarx-2 with a ThunarVFS-free API. All plugins shipped with Thunar itself need to be updated. ThunarVFS will be moved out of the Thunar tarball and into its own, so that applications depending on it will still work. And then we only need to fork the HAL based volume monitor from GVfs and of course thunar-volman needs to be rewritten on top of GIO ...
Anyway, just a few moments ago, I did something I thought is worth sharing: I browsed one of our servers remotely over SFTP! Mounting still needs to be done from the terminal but after that, you can simply type in the SSH or SFTP URI and it will display its contents. Thumbnails are generated over D-Bus. Here's a screenshot:
Thunar can now mount remote URIs on demand:
FTP browsing also works: