I seriously doubt it would help Pi very much, primarily for two reasons. The first being that, if you need that much mass storage, SSDs are now cheap enough to just use an SSD (very recently I bought a 500GB Samsung EVO 860 for just under $90, for instance), so there is no need to cache from an HDD to an SSD. Note that the technique dates to a time when SSDs were (a) expensive, and (b) didn't have much capacity. The second reason is that data transfer rates for mass storage devices on a Pi are limited to USB 2.0 speeds, nominally 480 Mb/s. With overhead plus the need to convert to and from SATA, you're going to top out around 30-35MB/s. Since modern HDDs can transfer data faster than that, there is little or nothing to be gained from caching, and quite a bit to lose, since you'd be transferring data from the HDD through the Pi to the SSD, which will drop the transfer rate to around 17MB/s as it all takes place on the same USB bus.
Now *if* a future Pi has USB 3.0 (or--better yet--USB 3.1), then you could transfer data faster than the HDD could supply it and putting selected data on an SSD could--in theory--speed things up. (I tested a WD PiDrive on an type of SBC that has a native USB 3.0. Transfer rates jumped from around 35MB/s to 80MB/s. In theory, USB 3.0 could go as high as about 500MB/s. In practice, I saw speeds around 125MB/s with an SSD but that board had much less capable ARM cores than a Pi3B has, let alone whatever one might expect from the next generation.)
tl;dr Not going to be of be of any benefit on a Pi...and it'd be a real pain to get working.