Hopefully a quick one:
How can I make a Playing Technique have an opaque background, so that it masks out things like barlines?
Font Styles don’t have background colour properties.
At the moment, I’m afraid this can’t be done directly. You could hide the real playing technique via properties, then add a Shift+X text item with an erased background in its place.
Hang on… if you can’t do opaque backgrounds for playing techniques, why is there an Engraving Option for the padding around PTs with opaque backgrounds? 
A bit more digging, and it turns out there’s an “Erase Background” switch in the Engraving Panel. Job done!
I can’t even keep up with the things I’ve done myself. I need a(nother) holiday…
Been anywhere nice? 
I’ve not been anywhere at all…!
Can’t say I’m surprised, though I’m empathetic - four days in Edinburgh over Christmas didn’t really cut the mustard here, either…
Dorico 3.5 Pro still does not offer the background colour of play technique text, and we should use system text by pressing shift+x for it.
Am I right?
You can erase the background (ie. opaque white) of a Playing Technique, but you can’t make it any other colour, like Red, or Big Mango.