Hi,
You shouldn't have any issue locking the rears with a hydraulic handbrake lever - that's really all they are intended to do, but you do have to keep a working cable system as well which confirms to MOT standards or it would be illegal.
If you use discs at the rear, then the HHB will be very worthwhile as generally the handbrake is poor on discs in my opinion. But on Sei drums, a well set up and adjusted system works perfectly well without lots of effort.
I've done a couple of autotests last year where there must have been 15 - 20 full blooded uses of the handbrake on a single test, and I never found mine lacking, but secretly I know I would love one as they look the business if fitted neatly.
Post a few pics if you get it done