Reading the instructions provided in the book it appears that it would be possible to headspace correctly w/o the gauges. BUT and this is a big "but", I spent some time today reviewing newer BLR's (why I don't know, this thread just intrigues me) and for the life of me I can't figure out what the above linked book is talking about with an "unmeasurable gap" on page 55 picture #4. Perhaps the instructions are for an older steel BLR which may have a different bolt assembly than a current alloy BLR. Perhaps this nebulous gap would become more apparent if the gun were disassembled.
So, while it might be possible to do it w/o gauges, gauges would still be advisable.