There's an open Github issue where the author clarifies that his own code is BSD, and he puts the license of each file in its header.
> The MH development repository has moved from https://github.com/glguida/mh to https://github.com/mhsys.
> Now that we have multiple external projects, like rumprun-mrg, and a strong interest in adding more ports for the mh userspace, it make sense to have a place to have all MH related code.
Thank you for your interest btw; this was completely unexpected and a rather pleasant surprise!
Development is currently stalled mostly for personal reasons, but I am more than available to help and answer questions to anyone interested in it.