32 bit userspace with a 64 bit kernel is absolutly supported by the kernel developers
I don't think either debian or ubuntu include the option to do this in their installers, but since the packages are available if you force overriding the checks you can install the 64 bit kernel on an otherwise 32 bit install (and it will even leave your old kernel available to boot from)
will you get some people who question why you are doing this when you ask on mailing lists? yes. will you get people who are doing this on their systems when you ask on mailing lists? yes (not always at the same time)
are you talking about paid support for either of these? if so you would have to ask the support orginization. if they are any good (and are charging you enough to really offer support) I would expect them to do so.
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