I haven’t seen a clutch master mounted behind the firewall either, but don’t see any reason it wouldn’t work with some creative engineering. Here’s what mine looks like with a Wilwood master mounted on the passenger side of the booster:
This picture shows the inside linkage:
Doesn’t seem like it would take much effort to fabricate the bracket you described and move the cylinder inside the cab. I would definitely want to use a remote reservoir. The risk of spillage on the carpet and poor ergonomics would make filling, checking. and bleeding very uncomfortable.