What I want from a hybrid drive is one that fits in a single space and connects as 1 drive, but presents as 2 drives. You can then use the OS (and / or user) to decide what performs best where. E.G. the OS boot-up and programs can go on the flash. Likewise the index files for things like Adove Bridge and Lightroom, but the bulk data storage can go on the disk. If the user is not so knowledgable about these things, the OS / applications can work it out for them.
Trying to get the disk to do it for itself just sounds like a bad idea to me; it has no idea about filestructure (and doesn't need to), so all it could do is shuffle individual sectors between flash and disk as it works out which sectors are used most / least often. Does it write everything to the flash first assuming that if it is recently used, then it needs to be faster? If I open a file change it and save it again, how does the disk know that it is the same file, and that I access it often (it could try to 2nd guess from trim commands, but how synchronised are they?)