I'm not sure exactly what you have in mind, but here are some random thoughts. If you are going to glue things onto the tank, it seems to me that they should be non-live things. Pillow moss is usually alive when you get it, but is likely to die no matter what kind of glue you use to stick it on. Peat moss is a dry brown powdery substance... i'm not sure this is really what you mean.
Could you glue on the cork bark and rocks and then hang some "drapey" looking dry moss or silk plants from it? Perhaps it wouldn't need to be glued in place. Live moss won't do well unless it's on a damp substrate, so I don't think that will work.
As for glue, aquarium silicone is the most easily available. You can use a hot glue gun, but it's not likely to hold as well. If you want to get really serious, you could use black expanding foam (or white expanding foam and paint it).
One other random thought... you can mix together aquarium silicone with peat moss or coco-fiber to make a natural looking substance to fill small spaces.