not a bad idea.

i've been digging the bins at auction and antique houses, but most of
the braces are broken, haven't found any good ones, and the Scot in me
won't let me pay for a new one...

Couple of suggestions....
Get an adapter for your speeder wrench and weld the chuck to the
adapter so you can just pop on the chuck when you need it... that way
you still have the speeder wrench for other uses.
Secondly, go to some of your local "yard sales" and you probably will
find a brace laying on the table for a buck or two. They are very
expensive if bought new.
Ken.