Aren't these parameters typically constant throughout the duration
of a network connection (and even the uptime of the machine) and thus
once determined, could they not be set globally for all users by the
system administrator, preferably in some sort of configuration file.

I would think that the typical user would not need to be aware of these
settings any more than they would need to know the machine's gateway/
netmask, etc. settings.  They could of course override the systemwide
settings if desired.

I suppose the drawback is that each application that used tcp/ip and
supports the ftp/http_proxy setting (e.g. ftp/netscape/lynx) would have
to be made to read that configuration file.

Even better, is there some way to push this proxy concept down into the
tcp/ip system so it is transparent to all applications?

I don't really know the inner workings of this and am probably way off,
but it just seems strange to have to set these same environment variables
for all user processes.