>since you can't render only a group of jit.gl.gridshapes together >and have them capture to a single texture (as far as i can figure >out)
This is not entirely true. Please see:
Recipe 24: GLFeedBack.
This gives a technique for using the "to_texture" message for capturing a scene to texture. I would imagine that using jit.gl.sketch is probably your most efficient option anyway, so it sounds like you are doing fine. I would love to see the results.