Hi Nick,
Yes the Fermi coordinate uncertainty left a lot of sky to search but one of the papers in Gary’s reference describes how this was managed by one of the teams.
http://iopscience.iop.org/article/10.3847/2041-8213/aa8edf/meta
Remarkably the combined data from the three operational GW detectors constrained the location considerably (and more accurately) compared with just the Fermi data. (fig 1 in the above paper). The DLT40 supernova survey team then prioritised imaging their 20 target galaxies which fell in that region.
Robin