It seems not possible to set the default value for a specific content element. PageTS reference doesn't mention such an option.
It is possible to manipulate the options array to some extent: See here for possible actions: http://docs.typo3.org/TYPO3/TSconfigReference/PageTsconfig/TCEform/Index.html#pagetceformconfobj
You could experiment with the option itemsProcFunc but I have no idea if it'S useful.