The problem with producing a billet for the RNR from the WMO is that, in many ways, 2DD is quite unique - he's an ex-matelot, with the appropriate training, able to go to sea for a bit. That isn't (at the moment) the structure of the RNR. For better, or for worse, the RNR replicates very little training the RN does. Creating a T45 (RNR) billet or billets would be quite a cool way to break the RNR.
Indeed, however a Sea Res rating could still fill a gap on a 45 and make themselves useful. Get the Sea Res guys into the duty watch roster covering gangway duties in order to free up the RN lads. Likewise Sea Res can't do anything in the Ops Room, but we can relieve the people manning the Jimpys on the upperdeck so they can fill the gaps down below.
Look at what other jobs people do onboard, bridge watchkeeping being the obvious one. You learn it on the job, they just stick you on the wheel and watch you carefully until you are deemed fit to crack on. There is no reason why Sea Res ratings couldn't do this, again freeing up a better trained and more qualified RN rating to crack on in the Ops Room.
Stuff Sea Res could do:
Buffers party, RF, gunners party, gunbay crew, boarding party, SSEP, BM/QM (at sea and alongside), DHP, potentially even stuff like Navs Yeo, it's only a 2 week course after all and of course chipping, painting and scrubbing out.
In an ideal world I'd like to see the RNR used as a pool of semi skilled manpower to deploy on ships for short periods to fill manning gaps. The DEPCOs bid for RNR bods, an email gets sent round everyone who's in date for ISSC and an APWT and you volunteer. Everyone knows you learn more onboard than you do on course anyway. As people start spending more time on ship, the RNR will become more skilled and more useful to the RN.