Since all felting involves getting the microscopic barbs of fiber (usually wool) to latch together through agitation, you can needle felt into / onto anything that
- allows the needle to punch through without breaking
so that the fiber you are felting can latch to itself ie netting, silk, cotton. I find the looser woven base fabrics are easier and more effective, especially for needle felting
- is wool so that the base layer and the wool you are felting anchor together
As the felting process pulls the roving / wool together, MOST woven fabrics, when felted, will gather or bunch up. The more tightly woven, the more the gathering. This can be artfully used. Also, you will likely see small needle holes as well. Again, this can be of creative use. I have seen photos of spectacular jackets made this way, where the felting and resultant gathering make a wonderful texture.
Debby is right when mentioning laundering. You would have to REALLY get the felted wool to lock together if you needle felt a design onto a garment. Machine needle felting makes a much stronger bond, due to the sheer quantity of needle punches a machine can offer. Wet felting involves soap and water, but I think even still -- say if you were to embellish a shirt or jacket -- you would need to be careful about laundering. In wet felting, the use of water and soap loosens the tiny barbs initially so that they can then better hook together. I have done a wet felting / needle felting process with silk scarves, for example, and some of the finer detail has loosened.
I agree that one approach would be to felt whatever elements you want to use in embellishing the garment, and then stitch it on. I think it should then hold on really well.
For technical clarity:
A knit or woven fabric which is then "felted" -- ie tossing it in the wash -- is actually called "fulling." Yes, I know there are books written about knitting and "felting", BUT for purists in the felting community, felting is ONLY when loose wool/fiber is made into a fabric without being first "structurely" interwoven aka knitting, crocheting, knotting, weaving. The end product most often looks and feels the same, but the technical process is different. I have only just recently encountered this precision of terminology, but it may help when you go looking for resources or if you ever decide to show your work.
Hope this helps! Looking forward to photos of your projects.