Any way to receive message, being a process on SYSTEM user

Question

So, is there any particular way to do this in Windows?

There's a service, that autostarts under SYSTEM user at every boot. By default, as stated at To schedule a task that runs with system permissions, any process, started on SYSTEM account, can't receive any information from other processes, i.e. using PostThreadMessage or SendMessage. ChangeWindowMessageFilter doesn't help.

So now, I have to do something like:

1. Save data to local users directory (that's done by client app).

2. Server every n seconds searches for a client process. If exists, server gets username, that started client process and in username's folder collects data.

The thing is: client needs to send little portion of data (< 50 Mb) to the storage. That could be done only with elevated permissions. So there's an elevated server that accepts this little portions of data.