Yes, silver solder is acceptable
The ATF procedure for measuring barrel length is to measure from the closed bolt (or breech-face) to
the furthermost end of the barrel or permanently attached muzzle device. Permanent methods of
attachment include full-fusion gas or electric steel-seam welding, high-temperature (1100°F) silver
soldering, or blind pinning with the pin head welded over. Barrels are measured by inserting a dowel rod
into the barrel until the rod stops against the bolt or breech-face. The rod is then marked at the
furthermost end of the barrel or permanently attached muzzle device, withdrawn from the barrel, and
Never done it, but I'm sure someone has fluxed the barrel threads, drilled a hole or two in the side of the hider and "wicked" silver solder into the assembly with a torch.
BTW, a crush washer serves no purpose when it's going to be permanently attached. If the hider needs to be timed, you're obviously going to time it before you weld or solder.