Bagua specifically? Never, although working with the ground is pretty basic in a lot of Chinese arts. There are various kinds of standing practices which can help a lot with getting the ground path - and the more you get it the easier it is to feel when other people don't have it (which is quite common). My experience has been that it's not that hard to get the basic sensation, but of course things tend to fall apart when things get moving - it takes some time.
Shioda talked about it a lot, and quite clearly, as in "In order to generate Kokyu you must stand as if you are making your big toe sink into the surface of the ground." - he often spoke about the importance of bringing power from the ground. There's a video clip somewhere where he says it straight out, hesitates, and then changes the subject.
Ueshiba spoke about it in very classical Chinese terms, Heaven, Earth and Man. The basic model, using the same terms that Chinese arts to describe power from the ground, in the same contexts.
Daito-ryu folks talk about it too, as Keisetsu Yoshimaru points out in this article
, so I guess we know how it got to Ueshiba.
Yukiyoshi Sagawa, of course, thought that building the lower body and the legs particularly was important.
I've heard two of the senior Japanese students of Seigo Yamaguchi talk about the importance of keeping your intent on the bottom of your feet, for the same reasons.
But...here's an interesting story as kind of a thought experiment:
Mayor General Masatoshi Matsunaga asked Sokaku Takeda at a seminar "Is Aiki possible if there is nothing (like the ground, or the floor) supporting your body?".
Takeda has 5 or 6 soldiers lift him on their shoulders - you've probably seen similar demonstrations, this is the one where the people lifting all collapse and end up pinned. in the case of this story, the person on the body apparently ended up fainting.
So...the ground power is an important thing, but not the only thing. Of course, it's still nothing to sneeze at.