Can I use a 6mm threaded bolt instead of a bleed valve for hydraulic brakes? I've accidentally lost the bleed valve in a front-brake "bleed gone wrong" afternoon and was wondering whether there's anything detrimental in just using a 6mm bolt instead for the time being?
Well you won't be able to bleed your brakes for one thing.  You can buy bleeder screws at auto parts stores.  I've never bought one, but 6 mm sounds like a normal size.  I'd check there.