Well I’m going to answer my own question, since I got it working.
Turns out you need to set an undocumented environment variable before creating an OpenGL context in order to get multicast support. This probably serves as a switch selecting between regular SLI and ‘VR SLI’ (aka multicast). While that makes perfect sense technically, I would have expected to find this small but crucial bit of information mentioned at least somewhere in the docs.
Anyway, for reference, the envvars are:
Setting either one before creating a context will make the multicast extensions appear in the extension string. Without at least one of these envvars set, the extensions won’t appear even if the hardware supports VR SLI and everything is set up properly in the control panel. Standard SLI will be enabled in that case (which is pretty useless in VR).
As an interesting side note, setting the former envvar will also enable the NVX multicast extension on one of our AMD Radeon based dev systems running the latest Catalyst drivers.