I don't think it would be easy. There's no automated keywords or anything.
The only way I have been able to get this information out of the system is via a JSAPI call , which can give you a list of ID's (users or groups) - there's no keywords to use. You can configure the groups to not auto-expand, I've used this before when I was wanting to easily audit security access for sites.
So if I were to do this (conjecturing here) I'd customise the logon design (or a paint layout with conditionals, if that's your preference) so that if not have access, need to make a JSAPI call on page load to get all the group ID's, then pass that list to an asset listing (ajax) or another set of JSAPI calls to get the groups names and then display them. Cons are:
- Onload operation (some SOE's do not like doing this).
- Depends on whether other people have access to add/remove groups and what naming standards they use.
I would not recommend this for a public-facing site, but if the user is already logged in but just happens to not have access to this page (i.e. User logged in = true but read-access=false), I think should be ok.