There is nothing stopping a GFPT qualified pilot from doing a nav ex anywhere that an 'approved' instructor approves them to go. This is how CPL training is conducted in a lot schools i.e. GFPT directly to CPL.
If you had done a thorough inspection as any licensed person should then I'm sure you would be able to say that you exercised your duty of care with respect to signing off the MR and that no undocumented defects were evident when you signed it.... but that is no guarantee. I wouldn't want to be signing it unless I was covered by insurance (e.g. under an AoC)