I don't think there should be any problems if you are several feet away from
the fencing wire, but you do need to be careful to not unbalance the
feedline. I base that conclusion on my working (when the world was very
young) at various point to point hf transmitting and receiving stations
where open wire lines were commonly run parallel to each other for very long
distances a few feet apart. To avoid the problems related to a tuned line,
you need to make sure that the line is flat - that means a balun at each end
that properly matches the coax to the balanced line. As you are proposing
450 Ohm line that means a 9:1 balun which is readily achievable.
Whether the exercise is worth it is another matter. I have a pretty long
transmit feedline and use surplus (free!) 75 Ohm 0.875 inch hardline, with a
50-75 Ohm transformer at the shack end and the antenna matching adjusted to
present a 75 Ohm impedance to the line. The losses are extremely low at
160m, it was easy and it was cheap!