Ignoring rugby league, where this is a well-known quantity, I was thinking about specific athletic skills required for different positions in rugby and in what sports they could be found for recruitment.
For props, you want both upper and lower body strength. Explosive power, but with some agility. And for them to not be muscle bound. I feel the obvious ones like power lifting and weight lifting would not be agile enough.
The sports I would target for rugby recruitment would be hammer and discuss throwers.
You want hookers to be more agile than props, so I would choose javelin throwers.
Locks need height, flexibility and hand eye coordination. One obvious sport would be basketball, but I would also look at AFL forwards (like Barry Hall), volleyballers, and pole vaulters.
For numbers 6 and 8, you want hard tacklers, guys who are good on their feet, are big and strong, and have reasonable pace. The standout for me would be college gridiron, players that do not make it to pro level. In particular, the Safeties and Tight Ends. In general, these guys are about the size and pace of Jonah Lomu.
For number 7, you need strength, flexibility and courage. I would look at gymnastics. These guys would have no problem getting off the ground quick, and would be able to get into position to pinch a ball.
At centre you need a strong runner, who is able to break tackles and go over the advantage line. From gridiron again, I would look at running backs. From athletics, I would look at decathletes.
On the wing you want pace and lots of it. This would obviously suit sprinters from athletics or wide receivers in gridiron.
For fullback, you want someone to be able to catch high balls, kick them a long way, and have good pace. My pick would be AFL players, probably those playing in the midfield
I have not been able to work out the best sports for scrumhalves (halfbacks), flyhalfs (5/8’s), or inside centre.
Maybe others can work these out for me.
These athletes could be put into the rugby sports academy for 18 months to see if their skills can translate to rugby.
It would be at a small cost. There are, for instance, plenty of really good basketball and gridiron players that do not make it to the professional leagues and therefore do not have many other alternatives.