I've helped several people here in the last year with no reverse.
In a case identical to yours (installed a supposedly good but long sitting trans), I suggested that he put the rear wheels up and run engine and trans for 30 minutes in drive. It was a long shot, but his reverse did come back; so it did appears the teflon seals had shrunk and the spinning trans expanded them again. Here is that thread: https://ls1tech.com/forums/automatic...rse-4l60e.html
You are correct that a messed up boost valve would give you no reverse, but no one has ever reported that. The reverse feed through the valve body is minimal. Certainly a good idea to check that all the valve body and separator plate bolts are tight as Frank (PerformaBuilt) recommended; trivial to do when the pan is off.
I've bought several transmission off craiglist with "no reverse", bought used pumps from disassembled transmissions, and in all those cases the teflon rings were shot.
IIRC, everyone I have helped on this forum with no reverse also found bad teflon rings. When the teflon seals go bad, then often ruin the inside of the reverse drum.
The following thread is extremely long, but starts with "no reverse" and goes through a complete rebuild in which I assisted at every step. The teflon seals were indeed bad, but the reverse drum also ruined the stator shaft which then prompted the OP to buy a new pump, etc. Once you open the trans it can be a slippery slope down a path of many details and complexities. However the OP was finally successful. https://ls1tech.com/forums/automatic...n-rebuild.html
I include links in that thread to other threads about necessary tools and recommended books. Also recommended rebuild kits and options.