I have digest authentication configured for a VirtualHost which proxies SOAP requests. I would like my SOAP service to be able to know reliably which user is accessing it -- is there a way to configure apache to pass through the authenticated username as a cookie?
I suppose you cannot but, in any case, you do not need such a cookie.
You can find all the info you need inside http "Authorization" request header. Any web server should set this header (apache does it).
See: section 14.8 of RFC 2068.