You don't say what version of eepc you tried his with?
We have tried this with both version 6.2.1 and 7.0.0.
We have some HP notebooks with mSATA caching and everything is working fine when encrypted. But I first enable the caching in the Intel Software and then encrypt the disks.
The cache drive isnt visible in Windows disk management so nothing is done by EEPC. Btw we're using 7.0.2