I actually tried the markup above earlier (and retested to double-check), and it just returns a world map w/o markers. Fortunately, I took another look through the theme’s framework files, and came up with a working solution! Basically the portfolio is defined in an array as type => port and taxonomy => portcat, so by using the custom taxonomy parameter, I can now filter the custom post categories like so:
[mashup query="portcat=johnstown" maptypeid="roadmap"]
Thanks for working through this one, Chris. Hopefully someone else will benefit from my discovery 🙂