Action after releasing the bump button

Not that I know of.

I would accomplish this by using both of the fader buttons with a macro.

The Play button to trigger and Hold the submaster.

The Stop button to run a macro to trigger an effect, fade the lights, and then clear the effect.

Timing on Submasters: www.youtube.com/watch

A sub can execute macro, cue or sub (one or multiple). It will however only react to pressing the bump button, not to releasing it. How long you maintain the press is irrelevant for the sub's execute functionality.

Eos is sending the event of pressing the bump button as well as the event of releasing it through OSC. If you manage to capture and modify this OSC with another application and then send it back to the console you should be able to make the console also trigger something when releasing the bump button.

is it impossible to force eos to accept the osc command in show controls and perform an action?

osc command eos/out/event/sub/x 0

If you want the show control tab to handle your incoming OSC it has to start with /eos/sc