FWIW, I did find a workaround, thanks to @Reboog711, by including a transformRequest parameter like so:
resource = $resource(
"http://foo.com/service/:type/:id",
{},
{save: {
method:'PUT',
transformRequest:function(data) {
delete data.type;
delete data.id;
return JSON.stringify(data);
},
params: {type:'@type', id: '@id'}
}}
);