UI thread starvation and hal pins inside M remap

I've created a pcb fiducial detector within camView, I'll assume this is running in the qtvcp/UI thread. 

I've exported HAL pins from the camView widget so I can control this, then added manual control buttons on the UI, and linked these in the postgui_hal file.

I want to control this manual button from a remapped m code, so I've exported another hal pin to do so.  The problem is that when I set this pin from the m code, the UI/qtvcp thread is never serviced, so nothing happens.

What are some ideas or solutions here?

Answering this myself for the next person, it needs

import time
from interpreter import INTERP_OK, INTERP_EXECUTE_FINISH

def change_tool(self, **words):
# Perform actions
# ...

# Pause for 2 seconds, synchronized with motion
yield INTERP_EXECUTE_FINISH
time.sleep(2)

return INTERP_OK

I'll add to this, this mostly works, but not perfectly, so any help appreciated about the reasons.