When there is no line where there should be a line, and the home team is not going to put a line in, my standard thing to say is:
"Since there is an imaginary line, you gentlemen are going to imagine that I get every close call on that line right!". That gets a good laugh, but also gets my point across!
If you make all the close ones go against the home team, you will see a line by the next game!
I don't see any problem with having the PU take ALL fair/foul calls on the lines if there are no lines in the outfield.