The only reason channelers need gestures is a mental condition. It is part of the mindset of learning the weave. In the first 2.5 books, Nynaeve NEEDED her herbs to Heal. It was not that she was weaving her flows through the substances, or imbuing the medicine with Healing properties, it was because she BELIEVED that in order to fix THIS problem, you needed THAT medical treatment, and she did not believe deep down inside that her channeling was sufficient. It was not until they were captives of the bandits in tDR, when Elayne had a skull fracture or something and Nynaeve did not have her herbs and she was forced to do without that she was able to break that condition. It is like the block Theodrin described having. She could not channel unless there was a man she felt strongly about present. She broke her block when they replaced the man who sat in on her lessons with a similar-looking woman, and she channeled believing a man was present. Once it was proven to her that there was no man present, she had no problem channeling. It is the same with any weave. Channelers get locked into a mental habit, and that is what gives them trouble. Since Rand still subconsciously thinks he has fingers, for all intents and purposes, he DOES have them when it comes to channeling. Just as Theodrin was able to channel when she BELIEVED a man was present even if none was, so too can Rand.

And in any event, even if this was not so, he still has the stump. He can simply rip out a single big Arrow of Fire.