Just to let people know the outcomes of this story:
The public officials got back to us after six days by texting the same line as in the enrollment form "locals enroll first, foreigners can apply to left over places etc"
On a happier note we found a private kindergarten at an affordable price, so I am happy that he can attend that school for the next year or two. Its a better deal than we would have gotten in my home country, where there is no public kindergarten.
In the meantime I am going to send a letter to the major Eric Chu to point out the issue, and to express the hope that things can be made more fair. Affecting Taiwan's image as a fair society, discouraging foreign nationals needed by the economy etc etc I am not planning on going to go to the TV news, unless we come up against the same thing when it is elementary school time.
Thanks for all the good advice!