FG of commercial brew

Is there any reason I can't degass and measure an accurate FG with a hydrometer from a bottle of commercial beer i am trying to clone?

Hm, I'm also interested in this.

That would work. There are a few factors that might make it a little tricky, such as if the brewery in questions uses a very fine filtration, but it would definitely help point you in the right direction.

You are definitely correct that you need to degass the beer first. If you don't, you will get a false low reading. Take the beer and pour it back and forth between two oversized containers, do it with a flourish, and do it repeatedly... perhaps 30 times if you want to be sure of driving the max amount of CO2 out of solution.