Neighbor transactions can't be implemented as a mere bit. They have to commit to the transactions they're paying for cryptographically or miners would be able to manipulate them.

Neighbor transactions could be more efficient by committing to multiple transactions at once though, eg "hash the next N txids". 

 And in fairness, they could use a special hashing mode so at least the signature is used for both commitments without an additional N byte hash. But that is getting complex. 

 Reverse it, it's still one bit for multiple transactions. "The next tx is committing to me". 

 Doesn't change anything. The transaction still needs to commit securely.