@mpoelzl this actually isn’t possible to do from an extension. The pause until block is special and can only be enabled by turning it on in the pxtarget.json of pxt-microbit. With that being said, I’m actually surprised that we didn’t already enable this in the micro:bit editor; I don’t think we ever discussed it on the team so this might have just been an oversight on our part…
You should open an issue for it here:
I personally don’t see an issue with adding it to the default toolbox, but I’m sure there will be some discussion over what category to put it in.