Where x86_64 Haiku nightly is able to run privileged commands over SSH with sudo option enabled, the x86 edition of Haiku nightly appears unable to do this. Since Haiku nightly is primarily a single-user system anyway, I am working around this by disabling privileged in my Vagrant shell provisioning configurations.
As it happens, vagrant rsync also depends on privileged SSH channels, so x86 Haiku nightly Vagrant boxes are unable to synchronize guest and host folders. I opened a ticket at Vagrant to account for this, though I’m not sure how long it will be before they implement a workaround.