I tried putting the 615 card into the 5100 (instead of the 610 card) and there was no change in the information presented so I guess it is not to do with the JetDirect card but the printer instead.
The 5100 has the latest (8.23) firmware so I guess it is just not capable of doing what the 4200 does in terms of displaying consumables status via the JetDirect card.