Actually, it is possible to do this kind of verification. There’s just no UI for it.
Look in buddypress/bp-xprofile/bp-xprofile-classes.php, method BP_XProfile_ProfileData::save(). There is an action hook ‘xprofile_data_before_save’ that happens (you guessed it) before the save. You could hook a custom filtering function to this action and validate your data that way.