AIUI, the optimization of the protocol structure (from block templates to header templates) that was introduced in Bitcoin Core 0.10 resulted in the disappearance from the protocol of some information that was key to the stake calculations performed by PoS coins. So, until someone publishes a general solution, the upgrade path for nearly all Bitcoin protocol PoS alts is capped at Core 0.9.

To my current knowledge, there are only three exceptions: PIVX, Navcoin and Spectre. I don't know much about Spectre, I only found out about it while assembling this response. I can't argue with Max Guevara's opinion of PIVX, so that's not a solution - and anyway it's a Dash clone so that pins it to a variant of Core 0.11. Navcoin is a clone of 0.13 with some elements of 0.14 back-ported.

I've been investigating the possibility of removing the proprietary anon stuff from Navcoin and cross-referencing it back against Bitcoin Core 0.13 to create it as a vanilla PoS 0.13 to use as a new base for Beecoin. It's basically complete afaict (all the expected tests pass), I just need to work out how to kickstart a PoS coin.

I wouldn't bother mentioning it except for the fact that the Navcoin clone retained the original proof-of-work code and so a plain PoS version with the PoW re-enabled might be a potential candidate for a future Bitcoin Core implementation of Slimcoin.

Such an implementation would require a re-write of the Slimcoin PoW, PoS, PoB code and the emission control routines to use contemporary Bitcoin Core objects and methods, i.e. pretty much the creation of a new coin --- so it might as well be launched as one (from the perspective of whoever's grafting away on the code) - because it's going to involve a lot of technically demanding work.

Cheers

Graham