It looks like we missed the news until now, but the commercial version (at least on Steam, but probably Humble Store too) has been ported to SDL2 (based on a pre-GPL state apparently), according to this email by Ryan C. Gordon:
The above link has an attached diff by Ryan which of course doesn't apply on top of our current master, but stays relatively simple so it should be doable to apply it manually on our code base.
The Lugaru mailing list on icculus.org also contains a later email about a SDL2 port pushed in a mercurial repo on Bitbucket by Thomas Glamsch:
A first step could be to review the above patches, and maybe do a quick search to ensure there aren't other rogue Lugaru forks with SDL2 ports in the wild. Ryan or Thomas might be interested in contributing directly to this repository - if not, we can probably handle adapting their work to the current code base ourselves.