You can't. Not everyone will be running a pruned node. Of course you already knew the answer.
And we're back at this, ahem, "trusted" archive nodes business...
If your 'stash' is sufficient value then you have a very strong incentive to pay the minimal cost of keeping your node unpruned.
So I get to waste bandwidth on every derp that figures keeping blocks around is unnecessary?
And I'm supposed to keep doing that when blocks become increasingly large?
A new node bootstraps by contacting 100 random nodes.
Directly?
Full unpruned nodes will be maintained by decentralist ideologues like myself
Special, as in Ralph Wiggum special.