Solid copper wire for bridge ground?

I am no expert, but can't see why you shouldn't be able to do that, copper is one of the best metals to conduct electrical current and therefor ought to make for an excellent ground wire.

Shouldn't make any difference if the wire is solid or made out of a bunch of thinner copper threads.

Only reason I can think of why a solid wire potentially could cause problems is that it can't be pressed almost perfectly flat under the bridge as if it would be able to if it was made out of thinner copper threads, which might make the bridge unstable and/or uneven, but likely it won't cause a real issue anyway, the wire will probably just compress down in the finish/wood allowing the bridge to still sit flat on the body.

 

The reason braided wire is used is so you can fan out the braid to make a flat connection under the bridge.

 

I've done far too many basses this way. You will be fine. I am not going out to buy wire for a bass, I use what I have on hand.