I can't see why it wouldn't be legal as you still run boosted brakes, plus this booster is better than the factory booster.
From the top of my head here is what I remember, this may or may not be accurate, as I said I've been out of Escortopia for a few years and have been dealing with rad Japanese technology for a while now...
VH-44 is the booster (made by PBR from memory?), essentially all it does is eliminates the big bracket that the booster and master cylinder mount to, and it is mounted under the dash, so that you only have the master cylinder and the reservoir poking through the firewall in to the engine bay. I'm not too sure what happens with regards to brackets, brake lines and the big bar that is activated by the movement of the brake pedal to push the master cylinder, you may need to either make something for that/have it made, or there might be an actual VH-44 conversion kit for Escorts available...I never looked too far in to it, instead I went straight for the rad pedal box.
Not sure on pricing of the booster, but repro parts are normally not cheap unfortunately.
This Datsun has a remote mounted booster.

Hope I've been of some assistance