It is likely that I wouldn't ask here if "VSR how to" would explain how to set up such resolution or would clearly state that it is not and will never be a feature.
No, I don't imagine driver size that would accomodate custom resolution in VSR. And I don't imagine how custom resolutions are a feature, how VSR is a feature, but custom resolution in VSR is not a feature. I didn't stumble upon knowledge that would make it obvious for me.
From CRU tool I remember that there are specific slot groups and extensions for custom resolutions. It was first time I have faced those terms in context of custom resolutions. I guess they are accomodated due to optimization and probably for storing on small, quick to access, physical memory on graphics card. But still that doesn't answer why VSR doesn't recognize and use those resolutions. Is it hardcoded only to use few most popular resolutions like 2K or 4K?