If I know that an off-topic question can be answered on a third party website/board, is it OK to send the Asker there?
Yes, it is.
If something is explicitly off-topic on Stack Exchange, there is no harm in sending the asker elsewhere.
However, do not use this for self-promotion. Do not promote a forum of your own this way.
People usually mention other sites in comments, rather than answers.
And, of course, if something is on-topic on another Stack Exchange site than it is posted on, by all means flag to migrate, and leave a comment for the asker.