I've had a Belgian strong Ale in secondary fermentaion with the dregs of two different brett beers (Boulevard Saison-Brett & Ommegeddon) for three months.
How do I take gravity readings without damaging the pellicle?
It's growing, but still pretty thin.
Just go for it and hope it holds or reforms?
I'd leave it. 3 months isn't all that long in the life of brett. I just tried my next door neighbors beer that has been fermenting for 4 years, and it was amazing. Just give it some more time. When you do, though, you'll have to break through the pellicle and they don't always reform properly.