I think you need a definition of what the term 'groundfighting' is and isn't by popular use of the word.
Throwing someone on the ground and submitting them while on one knee is not groundfighting. takedowns and finishes are not groundfighting.
groundfighing is when your opponent AND yourself have more than feet/knee or hands touching the floor, and both opponents are FULL-BODY grappling for dominance to either get away to stand up and run or submit the opponent from the ground to the ground.
Does kata contain the body mechanics necessary to specifically illustrate full-body grappling principles? no. how do we know that? there are no karate kata which contain principles which can only be interpreted as on the ground-specific technique. how do we know that? there are no karate kata with a performers body touching the ground. for instance: you won't see principles in kata which could only be interpreted as defense against someone taking the top mount: http://www.youtube.com/watch?v=ksUGmFz3wrI
in order to show those specific principles in the form of kata, the performer would have to be on their back when doing so. there are no karate katas like that. nor has the principles been illustrated in a karate book prior to the 1990's. nor have I read in any interviews of any pioneering karate master or their students describe such principles or even hint that they are part of their kata practice. This was only a phenomenon post 1990. If someone chose to train that way, it was their own invention via other-art training and not handed down thru kata as such.
is there overlap to some grappling techniques? that is, for instance, can you perform an arm bar more or less the same way on the ground as you can standing...sure, absolutely. you can also perform an arm bar underwater, does that make the kata designed to show underwater fighting principles?
and why stop there...you can probably pull-off an arm bar while sitting down, or free-falling in the sky at 5,000 feet. so, yes, you can perform some principles that kata has and apply it in various predicaments. but it doesn't change the fact that kata was shown and was handed down shown while standing on 2 feet - that is, prior to some people feeling inadequite for not having groundfighting in their art - most likely after seeing an early UFC and being horified. lol
so now karate has ALWAYS had groundfighting principles within the kata?...riiiight. what a coincidence. Anyone have any pictures of any master or any teacher or any practictioner demonstrating defenses found in kata while on his back?
don't try to confuse 'training drills' with kata. ne-waza has been combined from judo and jujitsu into karate for a while (we had ne-waza and kakie drills in Goju in the 70's). but before the 1990's, nobody felt the need to force a reconcilliation of the drills to kata.
it would have seemed as silly as trying to reconstruct hojo undo movements from kata.
if you have groundfighting in your karate, thats great - but it's a supplimental. just as some aspects of judo and jujitsu training have been borrowed into schools of karate over the past hundred years, so too some schools choose to borrow from various modern grappling arts.
which is great, but to think it's always been there and handed down thru kata, is not only an attempt to invent history, it detracts away from the focus of what kata is suited for: inspiring a standing defense.