I feel like I used to know this from the early PinePhone days, but if so, I have since forgotten.
What is involved in enabling the on-screen keyboard? Is that a build-time decision, or can it be configured at run-time?
The touch keyboard is only integrated by the phone manager, which is runs on the PinePhone. On PC, you first need to build the touch_keyboard component and install/launch it manually. Be aware, though, that there is no coordination (yet) w.r.t. window layouting. I have merely given this a very quick try some months ago. Be prepared for a rough ride 