In my e-mu sampler there's a useful function called "latch mode" which allows you to press a key and release it, and the sample(s) will continue to play until you press the key again, then the release of the envelope is triggered.
I don't think that there's a dedicated parameter/switch for that. But a workaround is to plug a sustain pedal to the keyboard and set the loop kind to Loop>Rel.
(You could use the sustain pedal instead of that e-mu switch)
The whole point of the latch mode is not to have any pedal or key to hold until you want it to stop. I want to start my drone sound and leave it there for a while, until I'm done improvising, then stop it. The sts doesn't seem to deliver this... unfortunately...
The idea isn't bad in itself, but in a live context... I want to turn off the computer monitor and have everything follow my patch changes and/or keys and foot controllers, so no mouse... I'll figure a way... but not with sts, obviously...