External HPL 2 use?

[Statyk]

It's possible. Full Conversions are, in fact, a different game running in the HPL2 engine.

Not exactly, because it's still using things related to Amnesia. A Full Conversion is simply a more customizable custom story.

The HPL2 engine is a FG property, so if you decide to make a game out of it, a good part of the benefits should go to FG.

Hope your doubts are cleared now

First off, he'd need written and direct permission from Frictional to do such a thing and the only people to do this so far has been The Chinese Room. I wouldn't encourage selling anything a member makes. If you are REALLY REALLY determined, I would suggest asking for permission by Frictional before even setting anything up, but even still, I doubt they will say yes. I would just stick to making custom stories/full conversions.