I made some kind of a working Draft for a control App for this. It is called harbour-btvol (Show sailfishos:chum / harbour-btvol - SailfishOS Open Build Service) and can be installed from CHUM. It has it little Bugs and Issues (does not list current volume level, list of devices is not filtered nicely, no Icon, i wanted to get the BT battery level as well but does not work yet, etc.) and moste of the code is “stolen” from @piggz amazfish connection dialog - so credits to him. 
Nevertheless, if you accidentally or while connected to another device reduced your BT devices “internal” volume you can now check out this GUI to increase it again.
Also @attah’s dbus command up there I use (credits to him!) is listed as deprecated. So I don’t know if it will work forever
Btw: This is not more then a Workaround, the feature needs to be in SFOS. And probably it is all controllable by pulseaudio or something like that…