If I had to install a brake on a BLR, I would hold the receiver in the 4 jaw, muzzle on the live center, and indicate in the breech of the barrel.
Wouldn't be quite as precise as indicating in the bore using a gauge pin at the muzzle, but should work, and would avoid removing the barrel.
I'd shift the front sight ramp back before threading the muzzle. That way, the original ramp mounting screw hole is available as a reference when setting up the Forster jig.
Then again, I probably wouldn't do the job at all, because I really don't like working on BLRs, and really don't like drilling sight mounting holes in lightweight barrels.
Sweating on the ramp avoids drilling and tapping.