I use such type of inflator assembly.
It has dump valve near connection of thick tube to the bladder.
You can use it safely, because this type of dump valve has additional mushroom-type valve before controlled valve.
I had some attempts to drop air from the bladder by this dump valve...
You have to turn your left side in to the upper point, and than pull the inflator tube.
I can tell just one thing - it works. Nothing else.
But later I nevre use it
Some reasons...
Firts - using inflator controlls it is much more easy to do it.
Second - from my students-in-diving times I got "good" manner - nevre use drain valve on the inflator, because it can have broken pooling wire, and you can rupture the ribbed tube. If you may NOT to do it - do not do it
But as normal (and more universal) inflator assembly - you can use it, why not. It is good. And there is NOT additional failure point, because exaust valve is doubled, and as standard plastic corner, as dump valve corner - it use same plastic!
But in case, when you could be jammed in the narrow hole, one more additional way to have ability to drain the air - could be usefull. Just as a spare way
About compartibility - I do not know. It is standard "chineese" plastic.
I know it is not compatible with TUSA and with Scubapro.
But it is compartible with such types of connectors:
