Just configure the producer as a 1756-L61S. I'm pretty sure that I've done similar in the past - as long as you substitute a controller that's got the same effective communications path, it should work just fine.
If you had a 1769-L30ER producing a tag, you couldn't configure it as a 1769-L32E, because on the L30ER the ethernet port is considered part of the PLC, whereas on the L32E the ethernet part is part of a separate "daughter card". So the comms data to/from an L32E has an extra "hop" to make, from the ethernet card to the PLC. Whereas the L30ER just receives the message directly. So in that case, it wouldn't work.
Likewise if the producer was a new 1756-L80E, and was connected to the network via it's embedded ethernet port. The comms path for a L80E is direct to the PLC; whereas for an L71, it's via a separate ethernet module and across the backplane.
But L71S and L61S both use the same "path" - into your ethernet module in slot X, onto the backplane to slot Y, and into your PLC. At that point, I don't think your PLC will care what model of PLC is giving it the requested data, just as long as the data is in the location it goes looking for it.