You could try using a sendHttpGetRequest() in a rule:
Item:
Switch WOL_HTPC "Computer aus"
rule:
when
Item WOL_HTPC received command ON
then
sendHttpGetRequest("http://192.168.0.26/jsonrpc?request={""jsonrpc"":""2.0"",""method"":""System.Shutdown"",""id"":1}")
end
sitemap:
Switch item=WOL_HTPC mappings=[ON="AUS"]
I not 100% sure about the double quotes, but it should work.