Not knowing exactly what the configuration is makes me hesitant to reply. Proceed at your own risk, YMMV, etc
Having worked in aerospace for the first career (15 years), I came upon a lot of frozen allen head bolts. I found that some heat applied to the surrounding area would tend to expand the I.D. of the threaded hole slightly, frequently freeing the bolt. The trick is to work fast because once the bolt heats up you lose, since the O.D. of the bolt expands as well, negating the advantage. I would soak thoroughly, tap the head to set up some vibration (may free rust or oxidation) and then try the heat trick.
If the problem is that it's over torqued, the opposite could work. Heating the bolt could expand the length and give you the needed release.
In either case, don't apply too much heat as taking the temper out of the bolt or surrounding metal could be bad.