The HOST-RESOURCES MIB omits Linux tmpfs volumes as part of a long list of "not real" filesystems. Unlike usbfs, sysfs, procs and the rest, however, tmpfs is a "real" filesystem even if only RAM-backed. Many people use tmpfs for /tmp, and some embedded setups have / on a tmpfs. As a result I think there is a valid need to have HOST-RESOURCES report on tmpfs volumes.
I'm attaching a one line patch which will allow this.