Just as a data point if you book a lap infant for one direction on a r/t ticket they allow you to pick the bulkhead in both directions even when you do not have a lap infant. One issue we ran across is that we are traveling with twins and ended up booking a grandparent on a separate ticket to help out with the travel. We wanted them to be in a bulkhead seat so that we could easily pass one of the children off if we needed help (restroom, eat, etc..). They said they would notate something on their ticket, but could not provide them access at that time due to being on a separate ticket. It wasn't clear if they had been on our ticket if they'd have access to that seat or if they'd actually need a lap infant attached.