downloadGWC doesn't do reprojection, so you need to download GeoServer 2.1 and set up your remote WMS as a cascading WMS, GeoServer will then reproject your remote WMS from 4326 to what ever projection you need. Then use the built in GWC to serve out tiles from the cascaded layer.
This blog post http://geo-solutions.blogspot.com/2010/10/wms-cascading-in-latest-geoserver.html describes how to set up your remote WMS. After that it behaves exactly as if you have a local layer.