the tig torch focuses the heat in too small of an area to use it instead of o/a. then there's the flux part. I'd say just tig them on. if possible get some breezer style drop-outs, or the surly's for track. they're pretty easy to tig weld on.
traditional dropouts that slide into slots in the stays are made to be brazed. you add a bunch of filler then remove the excess until it looks nice. say you decided to tig weld them on. first, how are you going to do it? you could weld around where dropouts meet the stays but it's kinda ugly. if you decide to mimick brazing, meaning adding a lot of steel filler while you weld, then you have a lot of steel to remove which is harder to do than brass, and you would have been better off brazing them in the first place.