It would be good but I suspect its unlikely to happen - when NLW digitised the selection of newspapers that now form 'Welsh Newspapers Online', they didn't approach any of the Welsh county archives to suggest they digitise issues from county archives to fill gaps available in NLW. This was a major missed opportunity, because for some key titles, some county archive newspaper holdings are far superior to NLW's for some periods.
The ideal situation would be for all the copyright libraries to contribute to filling gaps in the range of OS sheets available in NLS and to make the NLS maps website as close to a one stop shop as can be achieved. Very regrettably, I can't see it happening however. Nevertheless, hats off to NLS for what they are doing!