If the review process is really cumbersome, then choose incremental version numbering:
*hg_version.next_significant_git_commit*
*hg_version* changes with next CL to go repo. It might be just next number (1,2,3...), or hg revision number (it have increasing order).
*next_significant_git_commit* might be just next number, incremented when something significant has changed, but not yet added to Go CL.

I would prefer version related to dates, but as you stayed - if it is a problem, then there is no choice.