[TYPO3-english] TCA :: how to sort records by label_userFunc labels?

[Roland]

hi everybody,

i use label_userFunc for one of my extension's tables to display 
meaningful custom labels for each record.

is it possible to sort the records by this custom label by default?

$TCA['table_name']['ctrl']['sortby'] and
$TCA['table_name']['ctrl']['default_sortby'] do not to seem to work for 
this task, do they?

kind regards

roland