Assume there is only one spot on your hilly land that you can pull in
your favorite FM station, 90 Mhz, over the competition, and only with
a horizontal dipole at a certain azimuth. A Yagi doesn't help, as
we are depending fully on the null.

Commercial FM stations are vertically polarized, cause most of the people
that listen are in cars. I have found this to be true in my homebrew antennas
(at least around here). You might try changing the polarization, and see if
that helps.

Not sure why you think a yagi won't help. A 3 or 4 el. yagi with a good F/B
and proper polarization will be a big improvement over the end nulls of a
dipole.

Using great sheets of metal for a dipole will increase the bandwidth, but
that is not what you are looking for. Using the great sheets of metal for a
screen reflector, or corner reflector could produce some big improvements.

73 Gary N4AST