Here's what I did. I'll post it to the ST forum, as it may be useful to others.
SmartThings Community Post: https://community.smartthings.com/t/release-working-virtual-button-dht-for-action-tiles/162384
1) Created a new DHT using the built in "Simulated Button" example.
2) Edited the code,
- adding the capability "Momentary" to it
- for some some reason the Command endpoint in the DHT were called 'push1', where AT expects it to be called 'push'
3) Saved, Published and Created a new button device based on this DHT
4) Tested it internally in ST by way of the Smart Lighting app, created a temporary rule to turn on a lamp when the vbutton is pushed; works fine.
5) Authorized the Virtual Button to AT, and now it appears in the library and it creates a working pushbutton tile for it.
I've posted the tweaked code on my github here: https://github.com/MaxVonEvil/SmartThingsStuff/blob/master/ATvirtualbutton.groovy
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