b) Override import, open (and other stuff? what?)
Don't worry about this; it's taken care of by the rexec module; each application will probably want to do this a little differently (certainly Zope has its own way).
I believe I heard way back that there was a lack of confidence rexec overrode everything that needed overriding - or am I getting mixed up with restricted execution?
c) Wrap or replace some of the existing libraries, certify that others are "safe"
This should only be necessary for (core and 3rd party) extension modules. The rexec module has a framework for this.
It looks to me like a and b are shared with proxies, and c would be different, by definition. Is there anything else? Am I on the wrong track?
I don't know why you think (c) is different.
Because with proxies you'd wrap with proxies, and with capabilities you'd wrap with capabilities. Or do you think there's a way that would work for both (which would, of course, be great)?
OK, fair enough.
--Guido van Rossum (home page: http://www.python.org/%7Eguido/)