Just curious if it is bad form or worse against the rules to use a silver dollar for a ball marker on the green? Thanks.

No problem, but I would also carry a dime or similar object to use when the marker is in line of sight of you playing partners. I use a military unit coin and it hs never been a problem for me. It helps when I'm checking the line from behind the hole on long putts.

What kvn said. You can pretty much use anything to mark your ball. I'll even stick a tee in the ground on LONG putts so I can walk around and still see the marker.