Answering myself, partially:
After a quick read: libsmbclient.h doesn't expose any datagram stuff. That means we'd need to import samba internal headers in order to use the relevant functions which I hope are in libsmbclient.so. Or maybe just make a header with a couple definitions for just the stuff that we need. What are the policies on such stuff?
readelf'ing it showed a couple nice ones like cli_send_mailslot. That's what we need, I hope :)
Jelmer: what is the stability of mailslot-related function API in libsmbclient?
I would do an dlopen to bind it at runtime, only if available, so that wine wouldn't get a samba dependency. Otherwise I plan on issuing a warning and reverting to the current-session-only mailslots. Is that a good plan?
Cheers, Kuba