Part of the problem is that there is an andragogy for teaching the internal, and it is found in Xing Yi, Ba Gua, or Tai Chi. It is every difficult to bring in concepts that can be "spot" applied into another martial art. Perhaps not impossible, but difficult. In my opinion, many of the cannon of aikido techniques have postures that get in the way, so one would have to change kihon to a large extent. Ikeda Sensei has not done that yet, but without someone doing it I'm not sure the grafting of the internal onto aikido will be successful. Another way of stating this is to ask the question, if you want the internal so badly, why study it second hand.
I agree that grafting internal training from other arts with different patterns of movement from those in aikido might be difficult. For instance the Chinese styles you mention (Bagua especially) use a lot of large-scale twisting movement which is not typically used in aikido.
I'm not convinced, though, that any reintroduction of internal training into aikido necessarily has to be to be an "import". What do you think of what Dan Harden, who I believe has a background in Daito Ryu, is teaching to (among others) senior aikido teachers?