The proxy listed here is is the build node cacher (e.g. ACNG), so upstream proxies don't necessarily play a role here at all--that depends on how your build node itself is configured to talk to the upstream repos, which may be through a proxy or not and is independent of this preseed.  Variablizing it further is totally reasonable (e.g. setting a variable as to whether we even use a cacher at all); I've simply deferred that to a later patch.  This is equivalent to how the static file does it today and can definitely be further iterated on.