Not Quite Impossible. Not perfect, by any stretch of the imagination, but some of it is doable.
Feel free to prod developers about absent documentation, that’s always been a common shortcoming with plugins in general. We do our best to document our own stuff, and even that has a number of significant gaps. Assistance is always appreciated.
Resurrecting Cauldron for the purposes of continuing to mash together the Bukkit and Forge APIs is a process fraught with frustration, and that is one of the reasons Sponge even exists. They just plain don’t seem to like each other. Whilst it may be possible to make a bukkit plugin hobble along on a Sponge server using some implementation of Pore, it’s still desirable to port it to Sponge so that it can run more efficiently in the native implementation and also make use of the interoperability that SpongeForge provides to mods in general. Obviously YMMV.
It seems that a major inhibition of porting to Sponge for larger servers is the need to rewrite a large body of custom code, which is how they stand out from the pack. It will take time for a larger body of public plugins to develop, once a few parts of the implementation have stabilised. Hopefully the imminent launch of Ore will help kickstart that process. I guess it very much depends on your needs and wants in a server.