I think your question hides a bigger problem: the lack of content designing people (please don't call people resources, it depersonalizes them) seems to indicate that your team is working in development silo's.
On a recent project I worked on the same happened. We had front enders, integrators and back enders.
I was a nightmare. Although front enders were constantly busy, the integrators were not, and the backenders didn't know what to finish first. It ended up in having 40 or more unfinished stories at one point in time with nothing finished at all (beside the problems in the team, we also had to wait endlessly for stuff to be finished by another party).
We had to estimate the completion of stories just to give the customer an indication of the progress. Your question reminded me of this. Yes, we could show mockups to the customer, but as long as he couldn't use them (finished, fully documented, tested and approved), they were of no value to him.
My advice to you: when the content designing people are starting the work on a story, let them pair program with some of the others. It might cost some time in the beginning but might definitely be worthwhile the effort on the long run. With 20% of the knowledge often 80% of the work can be done. It can get some pressure of the current content designers and create a better flow in the team.