You can do this. Might be a bit jank but would work.
Create watch tv activity in harmony that turns on the tv and receiver
Create a virtual switch in smartthings
Create another activity that just uses your virtual switch that turns on at activity start and turns off at activity end and select “leave everything as is” when it asks “when this activity starts”
Create a piston:
IF
virtual switch changes to on
THEN
wait 5 seconds
turn on Wemo plug
turn on your activity that controls your tv and receiver
IF
virtual switch turns off
THEN
wait 5 seconds
turn off your activity that controls your tv and receiver
Turn off Wemo plug
You would just turn the activity on that controls your virtual switch and that should toggle the virtual switch, which will fire the piston and turn on the activity that turns on your receiver and tv. You’ll have 2 activities running on the harmony but I think it’ll work.
Edit: tested and this works. Definitely jank but I can’t think of another way of doing it.