Simple question for simple minds  

Its part 91.  I'm on the HAI UPAC working group.  Take a look at all the operators doing powerline work then look up how many have a 135 certificates(almost none) The FAA sends reps to our meetings and never has the issue of needing a 135 certificate come up.

If you're having problems with the FAA, call HAI and ask to talk to Chris Marino, they always want to know when FSDO are coming up with different interpretations of the regs.

Now, the fact that you're asking makes it sound like you guys are new and trying to break in to the work.  Please read/apply the UPAC guide, get some help setting up your program and get some professional training for both pilots and inspectors on flying in the wire environment.

Good luck.