matt on 03 May, 2018 05:48 PM
It's something I've thought about, but I don't have the resources to bite that off right now. If I did, it would definitely be only for use within a caltopo map; I'm not setup to scale up/down based on (possibly high volume) requests from 3rd party sites.
MapBuilder rendering is fairly customized in order to get adequate performance, particularly since contours require a lot of data relative to OSM roads/trails. It would definitely be some work, even if I didn't provide an editor and just had people upload an already-completed CartoCSS file.