I also have a generic 3 axis CNC router similar to the MultiCam and cannot find a post processor.
Have you had any luck?
Have either of you taken a look at the post-processors provided by PTC support?
Though I didn't see anything specifically for the MultiCAM, there is an entry or two for a Shoda 5-Axis Router, and maybe some of the other machines that are there is a router but isn't indicated as such. Maybe you could try one of those to see if the output is okay for you.
Otherwise, you could
(1) Contact one of the companies that have been suggested in other posts asking for post-processors. They make these things all the time and might even have a solution already available.
(2) Write your own, by modifying a generic milling one. You didn't specify if the MultiCAM model you're trying to support is 5-axis, which would make things much more complicated, perhaps, but it might not be all that difficult.
I talked to the company that makes the post processor files and was surprised that they did not have one for the most basic and generic CNC 3 axis routers.
They said the cost is around 3000 dollars plus for them to make one.
If it is a simple 2-axis router, creating a G-Post for one should be simple. You just need to understand what the machine wants in terms of codes to drive it. I'm sure there are many simple posts in the library that could be used for a starter, too.
Start with a simple shape, like a slot, so you get straight and curved entities in the motion. Post the file to different post-processors until you get close. Editing the post files is easy, first did a post 1984. The post generation software was come a long way since then. With one CAD vendor, we were editing the TCL code because they did not have their new post builder finished yet.