you'll have to fetch them from tagdata yourself.
A combo of TMPL->assign_variables() and preg_match() on
the keys/tags you're looking for.
You can see an example in a small calendar class i made: https://github.com/GDmac/calendar_entries/blob/master/system/expressionengine/third_party/calendar_entries/pi.calendar_entries.php#L52-L68