I have "steamed out" a few dents in gun stocks - is my practice to first remove metal, then strip the finish - try to get down to raw wood - many black marks go away at that stage, but occasionally is dent that is still there - will not work for "gouge" where fibres have been removed - just "dent" where fibres have been compressed. I take sopping wet with water dish cloth - lay on wood over dent, then apply a hot clothes iron - seems that steam and heat will plump up the fibres and dent mostly disappears - I do not think I have had any completely disappear - almost always needs a final touch up with sandpaper, with scraper or whatever.
Some time ago, I looked into the idea of using hot oil to straighten a warped long stock - sounds like the old school British thing is really about warming up the lignin in the wood that "glues" the wood fibres together - an entirely different process than getting moisture into wood fibres - I think you can find Brownells video on line - old dude Jack Rowe shows old school way to "bend" a shotgun stock - cotton cloth, linseed oil and flame - much fires occur - have to get that heat deep into the wood - is not a "work on surface" thing like steaming typically is.