I've found a solution!
Jayrock is able to process GET requests. I stumbled about at http://groups.google.com/group/jayro...39d07a7635cc51 where Atif Aziz links to a good documentation of Jayrock: http://json-rpc.org/wd/JSON-RPC-1-1-...tProcedureCall
Just a few things to change on the project files:
1. Change on serverside the .NET [JsonRpcMethod("myJayrockNETMethod")] to [JsonRpcMethod("myJayrockNETMethod", Idempotent = true)] to enable GET requests and let jayrock handle the passing parameters.
2. Change your store's proxy 'url:' to '/path/to/yourHandler.ashx/myJayrockNETMethod' to let Ext know, thtt there is the location to pass parameters to.
3. Also change in the proxy config the 'extraParams:' (only if your 'myJayrockNETMethod' needs parameters), just assign in simple "key": "value" notation, like:
Ext will encode the parameters to something like '/path/to/yourHandler.ashx/myJayrockNETMethod?user=peter&site=marketplace'
If there are some special chars in the string, they will be encoded. Also a good place to experiment with url-encodings: http://www.albionresearch.com/misc/urlencode.php