A good git import tool would put the mercurial hashes into the commit
messages, so there is at least any chance of finding them.
We're in the process of converting the BRL-CAD repository from SVN to Git, and what we opted to do was use the git notes feature to stash the SVN revision numbers on the old notes as commits. In addition to documenting which git commit corresponds to which SVN revision (so we can make sense of out of older commit references) it turned out to be possible to define a git alias users can put in their git config to check out specific SVN revisions from the git repo. I'd imagine something similar might be possible for an hg to git conversion, assuming sufficient motivation to preserve the information.
I don't know if any of the existing conversion tools do that out of of the box - our conversion case was quite complex, so we ended up (mostly) custom rolling our own solution, but one of the key points that recommends git notes as a storage mechanism for that sort of information is the original commit messages and timestamps are preserved. Using those as keys, it is possible in principle to go back (even after the conversion proper is done) and assign the notes with the hg repository metadata to the commits without otherwise altering the git history.