That problem also noticeably cropped up when Mozilla switched to Phabricator for its reviews, and the end result was/is that there's now an official helper script for submitting commit stacks without squashing ("moz-phab").
Because that took a little while to develop after Phabricator became mandatory, though (and also because installing arcanist with all its required dependencies can apparently be somewhat of a pain, plus because arcanist has some further foibles like insisting to actually check out each revision you want to submit, which then subsequently can lead to needless recompiling), some people wrote their own solutions, which a) were ready earlier than the official solution and b) allow you to avoid installing arcanist at all:
- "Phlay" for Git (https://github.com/mystor/phlay)
- Mercurial already has the "phabsend" extension, which was then forked and extended and currently lives at https://bitbucket.org/KwanEsq/phabsend-moz/src/default/, though I think some changes might have actually made their way back into the original phabsend extension distributed with Mercurial (one problem with the official extensions as of a while ago was that it used a Phabricator API that didn't work for binary files - I think this has been fixed in the fork, but not yet in the official extension included directly with Mercurial).
(Also I think that there's currently work underway to make "moz-phab" work without a local arcanist installation, too).
So the takeaway from this is that there is indeed no good official solution for this, but "phabsend" or "phlay" might be a better starting point.