Although I'm joking (slightly), pulling off inverted submissions of various kinds is, to some degree, a mark of considerable mastery of a submission. The body position is typically more challenging and the mechanics of the submission must be built in to your muscle memory (even more than achieving the sub from the traditional position) in order for it to work.
With that I leave you with a solid walk-through of a classic inverted armbar from the guard from Renzo Gracie black belt Karel Pravec (Silver Fox BJJ, Saddle Brook, NJ). This is a very legit technique and I have a few teammates who have been very successful with it over the years. It is by no means easy and will require tons of drilling and a lot of trial and error (i.e., you will miss this a lot in training), but go for it.
Thanks to BeltQuest.com for the sweet vid.