I use it in the form of micarta to make gun grips for my 1911. It's pretty awesome stuff once you get the hang of making it. I'm sure it would work great if you rolled it into a tube instead of making a flat sheet of it. I'm just offering my 2 cents, I don't actually restore brushes but I do know a little about Bondo.
Do you know where I could find the all-purpose blend? Or should I not worry about it and just get some 'auto' bondo. It sounds like that would be cheaper than the epoxy putty I'm using now...
The problem with it is that in a humid environment, the integrity can be compromised using Durham's. Also, it can become loose when subjected to movement. Although, neither may be a problem if you're using it to build up a brush Especially a plastic or Bakelite brush where there will be epoxy on top of it...
While never used bondo before but I've though about using fiberglass resin to fill a hollow handle. Its used on boats and is certainly strong once it cures and works well in wet envionments.
I get the all-purpose bondo at Home Depot in the paint section. Stay away from Durhams, it's a great product but water deteriorates it quickly and easily.
Last weekend I filled an EverReady 100 completely with Bondo Body Filler. Yesterday I drilled it out with a 3/4 in. forstner bit at very slow speed on my drill press. The excess material came off smoothly, evenly in long curls. It was a very easy material to work with.