RPC has an RpcServerInqCallAttributes function that an RPC server can use during a client call to obtain the client's security context attributes and other information.

You must pass a RPC_CALL_ATTRIBUTES_V2 available only with Windows Vista and higher Windows version (or RPC_CALL_ATTRIBUTES_V3 availble with Windows 8 and higher), so something like this:

// the code depends on the minimal platform you compile for
RPC_CALL_ATTRIBUTES atts = { 0 }; // should map to RPC_CALL_ATTRIBUTES_V2 or RPC_CALL_ATTRIBUTES_V3

atts.Version = RPC_CALL_ATTRIBUTES_VERSION; // should be 2 or 3
atts.Flags = RPC_QUERY_CLIENT_PID;
RpcServerInqCallAttributes(0, &atts);
... pid is in atts.ClientPID ...

This can only be called from the RPC server itself, so you'd have to record all client pids somehow and add some API to the server to be able to list them.