peterlu Before 17.9 it was not supported with any of the constraints in where clause.
But now it can be combined with depth = 0 and conversation.featured = true in the WHERE clause but lithium does not support "conversation.featured = true " with other constraints in the WHERE clause.
But you can achieve it using two queries
SELECT id FROM messages WHERE conversation.featured = true AND depth=0 order by conversation.featured ASC limit ${count}
Collect all the ids in an array in such format so it can be passed in IN query.
SELECT topic.subject, kudos FROM messages WHERE id IN (${ids}) and board.id = 'ID'
It would fetch all the required posts in the desired order.