The current eZP rest layer supports clients that can not execute PUT or DELETE calls by allowing them to make a POST call using a "magic" field _method.

This is good, but there is an industry standard we could follow instead: allow the http verb to be specified by an http header X-HTTP-Method-Override header

See eg:
http://code.google.com/apis/gdata/docs/2.0/basics.html
http://msdn.microsoft.com/en-us/netframework/dd547388