HPL2 Engine download?

[Mudbill]

As an additional comment, I thought I read somewhere that someone had used the HPL engine for a project of their own. I might be mistaken though, or perhaps it was HPL1 which is open-source.

If you contact Frictional Games directly I'd say there's a possibility that they'll allow you to potentially purchase any legal rights to use their engine. Not a very big possibility, but one nonetheless.

You're probably best off waiting until the source-code is released (if at all) but even then, making something commercial out of it will likely require a paid license from FG.