The ISO32000 spec says what first class name values aren't listed there are to be available in the ISO32000 Registry but this is unpublished. Even if Adobe is the only company currently using additional names I do not see this exempts them from publishing them simply because they are also the Registry's maintainer. They are simply the first organization of the database. Any second class prefixes in use by Adobe should be provided also, according to Annex E. The application pdf should be a link off an html page <http://adobe.com/go/ISO32000Registry> redirects to, not directly linked, it seems.
Otherwise no names are reserved and developers of alternative pdf writers are free to use what names they like. A conforming reader has to treat any such unregistered object as null or missing also I would think, including Reader. Any future conflicts with a registration attempt by another organization would be in favor of that company, I'd think, whether it breaks Reader, Acrobat, or a plug-in author's products for either.