Bitmasks or seed along with public key in WOTS+ post-quantum signatures
The XMSS Internet Draft actually does not implement the original XMSS scheme but XMSS-T as described in Mitigating Multi-Target Attacks in Hash-based Signatures, by Hülsing, Rijneveld, and Song (PKC 2016). There also a security reduction is given that shows that exactly this is ok (using a SEED instead of a list). It also introduces keying all hash function calls (which is also implemented in the Internet Draft) to prevent multi-target (second-)preimage attacks. The drawback of using a SEED is that this step requires a (Q)ROM proof. The reason is that the standard model PRF / PRG security properties all require the seed / PRF key to be kept secret (while it gets published in the given scenario).