Default distribution point that isn't the Master?

So to answer my own question, it looks like the overarching Network Segment for all IPv4 addresses does actually work, at least on 8.72. This effectively provides a default distribution point that isn't the master.

However, this can be confusing due to an oddity I noticed while testing: a nested network segment will inherit the distribution point of its parent Network Segment only if it has *never* had a distribution point set for itself. If you do set a distribution point for the nested network segment, it will retain that one even if you later unset that setting. ie. It won't go back to the default, nor to the parent's non-default distribution point. That's a bit unexpected, and in light of this it's probably best to have a distribution point explicitly set for all your Network Segments unless you're sure they've never had one set before.

My clients with 9.21 will switch on the fly to any distro I set, either on a specific network segment or on a parent, in any of the scenarios you just described. So, you likely won't run into these issues after you upgrade.

Ah, thanks. That's good to know.