I've restored the original volume of one album i've encoded from a FLAC version that has replay gain tags. I've played a track of the OPUS version and the volume is much louder than the FLAC version that has replay gain. So when there's no replay gain tags, it plays at full volume.
I've also tested replay gain with OPUS in MusicBee. It does play with the replay gain and foobar2000 does not. When i remove those tags again and scan the volume with foobar2000. Both music players play at the R128 volume. I think this is the value that you're talking about that the BASS library reads. At the volume leveling section at properties it does say it's not calculated if only R128 tags are on the OPUS files.
I've setup the file converter to use the OPUS encoder from the foobar2000 encoder pack, which is version 1.3 from opus-tools 0.2. I've also disabled the permanently adjust the volume to the replay gain volume as i still want to retain the original volume when replay gain/R128 is off. But the feature does work as i use it for my music player and the volume of every track and album is even no matter if it's OPUS or MP3.
So i think this is a matter of a missing feature in the file converter then. If you're going to work on the R128 feature, i recommend you to let the volume leveling section read those R128 tags alongside replay gain tags so i know those tags have their volume scanned with R128.