How exactly would that work? The traffic generator would have 2 interfaces in the same subnet ? So you would want the loopback to simply loop frames around at L2 ? You wouldn't be able to do that with a single L2 I-SID, as the MAC learning will learn the MACs of your traffic generator ports at one end only; the MACs of both traffic generators would be learnt at the local end, and frames would never be forwarded to the far end.
I think you would need to use 2 separate I-SIDs. There is no way to bridge together two L2 I-SIDs via soft loopback, you would need some hairpin connection for that.
But then in that case, why not use 2 separate L2 I-SIDs, each on a different IP subnet, and on the far end, you just assign both L2 I-SIDs into the same test VRF, which will thus IP route between them. That way you can set this up via software alone. Now your traffic generator ports will need to be configured in the two different IP subnets.