It will not be good for a thread that was not self-moderated before to be changed to self-moderated by the OP. People will later abuse it. You will see newbies and other people change a thread to self-moderated and delete the posts that they wishes at anytime which good members will not support on this forum.
That is understandable but if the admin thinks it is a good idea then it should be implemented for only high ranks like Hero and onwards. If still the thread becomes a spammers paradise then the mods can delete the thread. I liked the idea as it has happened with one of our members on our the local board and he had to lock that thread for some time and start a new one.
The simple explanation already had been made here, you cannot do that and also people will take that for granted and begin to abuse it, the way you see some making posts and deleting their own self posts could not be as annoying as seing your own post being deleted by someone who is the OP to his thread and taking action on deleting what you have posted, so if you want to be self moderated to your thread, then make it right at the start.
Please read what I have written above and I do think there is always a way out for such abuse. Now, it all depends on the admin to if he is interested in this idea which as always won't be given any preference.