Preston - When you say breathing, I'm guessing you are not talking about a quick whiff while using, but rather being in an enclosed space with concentrated fumes. Breathing solvent fumes is never a good idea (unless the solvent you are talking about is water, but it's not). Filter masks are cheap and easy to use.
I suspect (but don't know for sure) that your "proprietary solvent" is some type of denatured alcohol. Depending on what was mixed in to poison it, breathing fumes could range from a "bad idea" to "deadly". If you can find a MSDS for it you would be better able to make a decision about whether it could be useful to you and safe to use under your circumstances.
Since you don't know what is in it and you don't know what to do with it, your local hazardous waste disposal site might be the best destination for it.
Just one persons (perhaps overly cautious) opinion.
Ed