I'm not aware of any instances where this has been raised as an issue by Shanwick customers so I would say that making the case for investment would be difficult. Indeed, at Scottish, where the OAC entry point is even closer, close liaison between crew, Scottish and Shanwick mean this isn't a problem in my experience.
In any event, taking into account how far in advance this would have to be issued, I think this may well waste airspace for other users [and hence create a fuel penalty as requests would have to be transferred to lower levels] as the system would have to allow a crossing "window" to enable Manchester tower to integrate these departures (if they could easily do this ?) into their traffic pattern, CTOTs, MDIs etc.
Bear in mind also that a significant portion of NAT traffic uses ORCA [datalink] to obtain their Oceanic Clearance(s) and this [usually] can be done as soon as flight crew workload permits following departure.
If you have information about problems that are being experienced let the Shanwick supervsior / watch manager know about it and the problem can be tackled appropriately.
RT