Oops, spoke too soon. I may be able to hide the TOC list item in the logical TOC. However, that doesn't hide the entry in the reading system's TOC for current Kindles (and even Adobe DE).
3.2 spec says:
To control rendering across all Reading Systems, authors MUST use the [HTML] hidden attribute to indicate which (if any) portions of the navigation data are excluded from rendering in the content flow. The hidden attribute has no effect on how navigation data is rendered outside of the content flow (such as in dedicated navigation user interfaces provided by Reading Systems).
MUST is the key word here, but apparently that's not true for Kindle or even for Adobe DE.
It's funny, because I can think of multiple situations where I want content to be not visible in the reading system's navigation entries.
Looking further, I see that linear=0 doesn't seem to hide the TOC item from the reading system's navigation system. (Maybe that was a legacy way to do it which was not really implemented?)