I have used some loads for training purposes just as you are proposing. In my case, I modified a NEF single shot 20 gauge by cutting the stock, installing a pad, and reaming the choke to .006" constriction. I loaded 3/4 oz of #9's with a .410 card wad inserted into the shot cup to take up space. I was also successful in using wads that I cut from a piece of foamcore board with a punch. The loads that I used are no longer listed in the manuals, but they were published data in the late 1980's. PM me if you need them.
By the way, the kids absolutely loved shooting skeet with the reduced loads. Each boy shot about a half-dozen shots, which was enough to satisfy them temporarily. Be sure to give them eye and hearing protection. We started with incoming targets on station #1. They were surprisingly successful. I think a lot of folks would tend to start a new shooter on station #7 with outgoing birds, but they tend to wait until the bird is out of range before pulling the trigger. The incomers tend to give the kid a chance to make up his mind to shoot.
Clemson