"Since not all coax jacks in the rooms are connected to a tv,
it's not just a matter of disconnecting one cable after another
from the splitter and seeing which tv loses its signal"
I can think of two ways right away. The way I would do it is
to get a small TV and conenct it to a cable outlet. Turn the sound
up high. Go in the attic and perhaps with a helper listening
disconnect one coax at a time. bingo.

the hard way would be to use a volt-ohm meter. disconnect all the coax
in the attic and jumper one outlet
across itself. then check in the attic for the line with 0 resistance.
....thehick