There is no such thing as object based upmixing. Object based means that specific sounds have been designed to be in specific room locations and will scale to what speakers you have available.
The upmixing done by Atmos, DTS-X, and Auro 3D from legacy two channel and multi-channel content just fills the extra channels but is not object based. You can do the same in JRiver by using JRSS to upmix to 8 channels and then use the PEQ and Mix Channels to combine other channels as necessary. For example, if you want an overhead channel, you could mix the side surrounds together for the overhead "Voice of God" speaker. This is really all that the other upmixers are doing. If you want two overhead speakers side by side over the main seating area you could mix more of the left surround to the left overhead and more of the right surround to the right overhead speaker. JRiver's Mix Channels is flexible enough to accommodate just about any layout.
Matt, I agree that JRSS upmixing is the best. What some people want now is upmixing to more channels than 8. I don't think it really makes a lot of sense in a home theater, though. Since Atmos and Auro 3D use completely different speaker layouts for channels in excess of 8 channels, I don't see how JRSS could be made to work with these layouts. Maybe you could come up with some 10 or 12 channel upmix with JRSS following the
Atmos layout just to make it easy for people. However, not many people even have multi-channel DAC's with more than 8 channels.