No, it wasn't merged.
or if not, is the merge still planned?
We still hope to merge it at some point.
If so, is there anything I could do to help speed up the merge
process (as of yet I don't have any experience with the overlay
mechanism or Emacs' C internals though)
AFAIK, the work on the branch is not finished, and it is not yet ready
for being merged. So the problem is not the merging itself.
At this point, I also doubt I'd agree to merge it before the emacs-27
branch is cut, as we already have too many potentially destabilizing
changes on master, and adding one more will run the risk of making the
Emacs 27 pretest even longer than I fear it will be.