That would mean attending a few of their early story meetings where he would present to them arc by arc his book (a few days of work), and wouldn't delay him much in his own writing, but I doubt Martin will participate all that much to the development of GOT if they overrun the publication of his books.

My suspicion is that he provided them with material with which to develop their own ending in this case, with freedom to use his characters as they see fit and all.

I think their understanding is probably that the series and books will go their own ways if this happens, though as I suggested in my other post, I wouldn't be surprised the option is also on the table for them to get access to far more than an outline of the last book is they choose to delay broadcast of the last season of the series until the book is ready for publication (within reasonable limits, such a clause would have a deadline after which they can broadcast the season even if the book isn't out).

They can't stop production without a massive financial impact and other problems, but to delay broadcast by six months or a year they can.

I think at the moment they're still more or less hopeful Martin will be far enough along in the writing of the last book by the time they adapt it that they'll be able to work something out. I also doubt HBO, the producers, Martin or his publisher are overly concerned with what might happen down the line several years from now. I think the more worrying scenario is the one in which Martin won't publish Winds of Winter in time for them to adapt it, but they no doubt have a far more accurate estimate of the likelihood of this happening than we do.

My guess is that WoW will be out at least a year before its adaption starts on HBO, and if they give it two seasons, perhaps mixed with remaining episodes from ADWD, and if they throw in the prequels mini or movies at some point to gain a year, the odds are good they can produce an adaptation of DoS rather than write their own version of the ending.