[darkgael]

A problem that you are going to run into is low pressure in the shotshell itself. The payload of rubber pellets does not provide much back pressure to help advance ignition.
I have loaded very light "chaser" loads to keep black bears off the decks adjacent to my home. (Don't have the load data by me but the charge was either Bullseye or RedDot). As I recall, the powder charge was a standard low pressure load from Lyman; the big change was the payload (Note: the general advice when loading shotshells is to use published data exactly as written as pressure excursions may result. This type of loading ignores that caution. Be warned.)
I used round plastic beads about the size of 000 buck shot.
They worked but I don't know whether it was because of the pellets or the noise.
You need to find a way to compress the charge a bit so that it can burn properly. I used an old fashioned Nitro card/fiberwads/ OS card/roll crimp to set up mine. The column was compressed with 30lbs pressure.
About legality. That I will leave to others. Where I am is very rural and use of this type of shotshell is not a danger to other people.