Cams for Graphic viewfinders can be made. Wander over to and use the help forum's search facility. Reading ads is a poor way to educate yourself.

I take it that you're a one lens, one emulsion sort of guy.

In a typical outing I use more than one lens on my Graphics. Since calibrating a Kalart for a lens in the field is OUT, I focus on the ground glass. Yes, I could have multiple focus scales, multiple sets of bed stops, use the Kalart (calibrated for the one true lens) as a range finder and transfer the distance measured ... In principle, that is. In practice, I use more than three lenses on an outing, so that approach is OUT too.

Stop looking for a magic bullet.