In the of the house?! Just kidding. I know you meant middle. I've encountered this twice in the 5 yrs I've been installing MTS' equivalent system.
The first house was brand new, and the owner had to have the builder come down and cut open the ceiling, as the copper drop and ground hadn't been run in yet
Come to think of it, I may have just left the fibre line coiled up in the demarc outside, so if they ever switch to fibre, they'll be in the same boat. This was before FTTH went live out here.
The second home, which I did recently, was a FTTH install. Thankfully they ran a cat5, ground, and fibre patch from the demarc to the panel. What sucked was having to terminate fibre outside in the cold. I bet the windchill was like -20. Maybe worse.
Anyway... If it is a straight shot, maybe one of the wires going to the demarc could be used as a pull wire, then pulled back with a string taped to it for pulling the fibre line. Technically, if there are copper services going into the home, they MUST be grounded at the demarc if terminated. We are not allowed to install without a ground, per CRTC. Apparently fibre drops can have a tracer wire in them, which we've been told we just have to pull back to demarc and coil up, thus no metal wire going in.
The ONT's used out here use specific power cables to and from the battery backup, so that might not work in your case.
As installers, we often encounter difficult situations like this. Rarely do we have to turn them down. Good luck!