I heard that LVM/bcache can be used in Linux to store the most accessed files if there is a separate SSD drive.With a hybrid drive (SSHD/H-HDD), can the flash area be manually used by LVM/bcache as well instead of the disk algorithms? 
Using the SSD area of a H-HDD drive with LVM/bcache
linux;lvm;bcache
If you format a piece of the SSD drive to be used as a bcache cache, then it will not be available anymore to store any other filesystem, obviously. But nothing prevents you from using the remaining part of the SSD drive as you see fit.This applies to for instance WD Black2 There are also other SSHD disks, like Seagete Momentus XT, where the cache is hidden behind the on-disk cache controller, acting in place of the bcache logic. In such case unfortunately you most certainly cannot access the cache yourself and manage it manually.