I don't think so. You a assuming that everyone who will ever post to p6p will know to add the keyword and also what keywords there are. This simply will never be true. If people want one big list then, IMO, it should just be a re-distribution list. This list should not change any headers and only forward mails that were posted via one of the other lists.
All replies go to the list the original came from.
If someone tries to send to the list directly, they should get a reply telling them of the different lists that exist and tell them to resend to the appropriate list.