I tend to practice the Taikyoku shapes from two perspectives (each having a continuum of form/training/principles vs. application):
1) How any technique is basically either a riff or chaining sequence of Taikyoku shapes, such that doing TKK shapes while fitting in appropriately with the energy another person brings will enable techniques to happen.
2) As a container for training internal strength - you need to have solid basics with regard to Jin, Ki, kokyu, etc - but the framework (with all movements being patterned on TKK which are themselves variations of ground up, gravity down, opening and closing the body) is comprehensive enough to imbue whatever IS practice you have into it.
I have worked with bits and pieces similar to this. However, i don't have the systematic build like TKK. I really like TKK, because it's closely related to aikido that folks won't object too much to its introduction. i think i will drop this on our beginners just for fun.
the discussion of ikkyo undo got me to think a bit which is always a dangerous thing, because my brain doesn't like to think too much. most folks, that i encountered, view ikkyo as up and finish with the down. they viewed the down as the finishing portion. whereas Ellis used the down as the starting point instead of the finishing point. i kept thinking of Tongbeiquan.
one of the IS dark lord of the Sith mentioned that aikido is a really good container for IS training. however, folks tend to focus on the container and not within.