Did you look at the Gemstone stuff that I reposted?  It sounds like that
system is close to your goals and it adds no new syntax (no dot syntax, no
colons, just normal message sends afaik).  And I think the security of it is
nice as well since it looks at the user compiling the code and then makes
the link at compile time.  If you file out your changes and someone else
files them in, it looks like that stuff would get rebuilt so the only
security problem would be if you ship the byte codes directly, but I guess
they probably have some solution for that.