Done.
Also done, implemented by Storeman 0.7.1 and 0.7.2-release2. Mind that I accidentally broke exactly this new feature in Storeman 0.7.2-release1.
TL;DR
Storeman Installer works fine now on any SailfishOS release (≥ 3.1.0, that is) and should continue to do so with future SailfishOS releases (e.g. 5.0.0.x, 5.1.0.y etc.).
Thus, as originally stated: