So far the best (by far) solution I've been able to find is to use ArcGIS to manipulate the elevation values prior to letting C3D at them... I created a fully metric version of the DTM, where the elevation is also in meters. C3D dutifully follows my coordinate system transformation and blindy converts the meters to feet. (and now that is okay.)
ArcGIS can be a great way to solve problems like this. Nice suggestion.
Global Mapper is another great tool for doing this as well.