Thanks all! Yup, confirmed - the bus had Amtrak California on the side and large luggage was placed under the bus. There was some space in the bins above the seats, but they were not very big, similar to the size of the ones on the Capitol Corridor train. There was no branding for the actual operator, although I did spot a WiFi name of "All West", and it looks like All West is somehow related to Coach USA. (There was also a WiFi network named "Amtrak" - neither WiFi worked very well unfortunately).
Just in case this info is useful, the bus was located at the NW corner of the station where there are a few bus parking places. From the train, we walked through a tunnel and then under a covered walkway to the station (H Street, I believe), and the buses were to the right - we didn't need to go into the station.
The ride itself was pleasant enough. Although we left Sacramento about 30-40 minutes late (waiting for another bus coming from Stockton apparently), they had enough passengers for two buses, and each bus only stopped at half the stops, so after stopping just at Rocklin and Colfax, we actually ended up in Reno almost on time. The Reno station seemed to be closed when we arrived around 6pm.