Can the Steyr bayonet mount be attached without removing the flash hider?

No...

http://bullpupforum.com/index.php?topic=5146.0

I was wanting to put one on an A1



 

Use a 19mm or 3/4" wrench (does not have to be thin type) to back off the A1 checknut. You may or may not need heat, probably not at this point if it's an A1.

To back off the checknut, you want to tighten it. Left hand threaing, so go counter clockwise while looking at the muzzle end.

If you have to, use a mini strap wrench for the A1 flashhider. You could instead slide a screwdriver or bar between the slots, but it may not turn out pretty.

Putting it back on I like to use rocksett.

ETA slide some sheets of paper between the wrench and checknut to avoid marring the checknut.