You are probably right in thinking the 30JDJ would be close. If Merrill used a full length 225 case, then the variants between the Merrill and the JDJ wouldn't be much different in performance I'm guessing. It is odd no measurements are around for the Merrill. But whether it be the same or not, you could still draw up your own, plug the numbers into LFaD and come up with starting loads. Guessing again, being that the .225 case has thicker walls and a standard bolt face rim, it is still close to the 30-30 capacity. So if nothing else, light .30-30 loads might be a starting point, without investigating it further.
I don't know what JD's dimensions are either, But if it was me, I would still send a drawing to Mike, say blow the case out to .010" taper, 35^ shoulder, with a 1 cal. neck on a full length case. Chances are it being a Merrill are slim, and I doubt it would be a JDJ either. But you could call it whatever you want, unless someone decides to dispute it.
Respectfully
Dave