How can I manually force a file of a Fusion Drive to the SSD? From what I understand, the implementation of the Fusion Drive is at the OS level (coreStorage). Yet, I want to know if it's possible to manually interfere in the management for a specific file or directory.

E.g. Most Photos in iPhoto are rarely used, yet I would like to push the entire directory ~/Pictures/iPhoto Library.photolibrary to the SSD.
I am aware this may not be the technical answer you seek, but it's the pragmatic solution at present. Hopefully, a non-buggy way to micro manage the tiered storage will become known, but here are things you can do today, out of the box. 


ensure all data fits on the SSD so that no tiering takes place. 
manually access these files to ensure the algorithm sees them as being read more than the rest. Using dd to read the files would be a good start and rebuilding the iPhoto library databases should also force a full read of the contents. It's not clear reads only will affect tiering in a "mandatory promotion to SSD storage" sense.