David,
Yes, when you start to dig deeper into the past, the religious/political considerations would certainly have affected recruitment from the rural communities of mid-Wales which were (and still are) relatively sparsely populated. And sons were needed on the farms for the families to survive.
A couple of days ago I was working at Fradley, near Lichfield which is not far from the route of the HS2 London to Birmingham rail link. I noted the vast swathe of high grade farmland and habitat that had been trashed for this new permanent way and remembered all of the furore of local people who vehemently opposed the project.
It must have been the same for folk in and around the Elan Valley back in the day, being evicted by the Birmingham Corporation and suffering a vast influx of 'foreign' labour as navvies etc.
Little wonder that recruitment was low and few sons were to make the ultimate sacrifice in South Africa and end up named on memorials.
Cheers Steve