Thanks for all the feedback. Mixing and matching what other answers that were given, I got this.
Mixing it all together a bit, here is a very very quick snapshop of what I'm going to do. Please note that I did this in 5 min, didn't take out all the gear...and only did the neck part for demonstration.
Take a simple picture picture of your item on a mannequin:
Take a picture of your t-shirt inside out on the same mannequin
Mask out the neck part of the picture you took in step 1:
Mask out the picture you took in step 2 and add some shadow:
Put your step 4 underneath your step 3, then simply stamp or heal tool where you need to get the result:
This is pretty time intensive editing. But it can give awesome results.