Ooh, that looks interesting ... and not just for ROM feature. They've packed a TON of things into there. Even SCRAM support!
I'm really intrigued by one of the planned features... "Hash upgrades to higher cost settings without knowledge of passwords". I can think of a way to implement that alone, but to pack that all in with those other features will be really impressive. And would be a huge boon to systems w/ infrequently logging-in users.
"Hash upgrades" have been added with the just-published v1 of the yescrypt submission to PHC. There is not yet an implementation of an "upgrade this hash" function (also, hash encoding needs to be finalized, including encoding of the upgrade count parameter), but now it's a matter of writing this additional code outside of the actual password hashing code (already capable of computing possibly-upgraded hashes).
Unfortunately, with memory-hard schemes such upgrades involve an efficiency loss in terms of normalized area-time, and there's a tradeoff between granularity of upgrades and efficiency. For Catena (another PHC finalist that supports these), the time granularity is 2x to 3x, with efficiency down to 33.3%+ of non-upgraded hashes (after many upgrades). For yescrypt, it's 4x to 5x granularity and 60%+ efficiency, respectively. (I felt that 33.3%+ is just not good enough - in my opinion, it's usually better to postpone the upgrade until 60%+ can be achieved rather than upgrade early and prevent it from ever being achieved. The next easy step would be 77.7%+, but the granularity would be too high.)
At the same time, support for ROM access frequency tuning (and thus for ROM-on-SSD, as opposed to ROM-in-RAM) has been dropped from yescrypt for now. The rationale is that builtin ROM-on-SSD didn't make enough sense without also having a ROM-in-RAM (in use cases for the former, the latter would also be easily affordable, including complexity wise, and would provide significant extra defense), and supporting two ROMs at once would be further beyond the comfort level for complexity of a hashing scheme in PHC (yescrypt is already somewhat beyond the comfort level). Support for two ROMs along with access frequency tuning might be re-added in an extended revision of yescrypt for specialized use cases at a later time, but this will be outside of PHC.
I'm really intrigued by one of the planned features... "Hash upgrades to higher cost settings without knowledge of passwords". I can think of a way to implement that alone, but to pack that all in with those other features will be really impressive. And would be a huge boon to systems w/ infrequently logging-in users.