I feel your frustration. I did not read your replies so I am sure that you have got lot of good advise. I will tell you that my daughter who turned 7 today still does this from time to time and has since she was potty trained. She will go months with no problem and then all of the sudden here we go again and it may take from a few days to several weeks to turn her around again.
I, like you, grounded her, took aways things, did charts, ignored it, talked till I was blue in the face, time out you name it I did it. Funny when I would ignore it she would always ask if I was mad at her. I would tell her no I just wanted to help her.
Her kindergarden teacher even worked with her. She did tell her teacher that she didn't like to go potty because she didn't like the water splashing. Honestly, it has been the hardest thing to deal with. When I would ask she would just tell me she didn't know. She would even try to hold it in. I told her that my main concern was that kids would eventually figure it out and tease her about it.
I also took her to the doctor to make sure there was not physical problem which there wasn't. He talked to her as well.
I finally put her in play therapy as there was a lot of things going on in our home. The therapist believes that this is directly related to stress. I will tell you that I tracked when it would happen and sometime it would be around some event and sometime it would not. Now we have started 1st grade and the problem began again. It lasted a short time and it is turned around once more.
I don't really have an answer for it as I now have a 7 year old (just turned 7 today) that I am still dealing with the problem from time to time. Fortunately, it has gotten less and less.
We have both prayed to God to help her with the problem as well. It think that helps her.
All I can tell you is to be patient. It is very exasperating (sic). Love is the best therapy. Talk to your child. Believe me ther is something that is causing her stress. It may just be the start of kindergarden or it could be something more. It will get better with time.