I am testing this rule in practice mode and have changed the rule to back the match favorite only when they are serving and it appears to be doing that.
Is it possible to lay the match favorite when the opponent is serving?
Yes, just change the rule to 'Place a Lay bet'
Then change the server condition to 'Selection NOT serving'
If you trigger a 'Fix Order of Market Selections' rule pre-off, that will remember that order for the duraction of the match
The rules can then all be applied 'By Position (Sorted in order of favourtism) 1