If you were required, for whatever perverse reason, to maintain a 70/30 portfolio in which the 30 is something other than equities, what would it be?

Ummmm....odd question.

I'd always go for maximum prospective real total return conservatively estimated, over the intended lifetime of the investment.
It does depend a little on whether this is a portfolio that it getting regular deposits, regular withdrawals, or neither. And whether the cash in/out is small or large relative to the balance.

So it's a question of what, other than equities, might give the best real return over the investment horizon.

But in general, the things that come to mind for at least consideration would be any or all of
* Inflation protected bonds, ideally in a mix of currencies, if the yield on offer isn't too awful. Early 2025 TIPS are yielding inflation + 3.1%.
* Cash. It has so many uses! This presupposes that you're willing to use it when the time comes, which breaks the 70/30 rule.
Either for an emergency or a compelling investment opportunity.
For example, junk bonds occasionally crash and give high forward returns. If you see 14% yields, go for it.
* Sometimes I've picked WFC/PL. It takes a nasty hit from inflation which hits both the value of the capital and the coupons, but even so it has given a real total return of around inflation + 3% in the last decade or so.
In recent years that's certainly not bad for fixed income.
* I've had good results with a hedged cash-backed put-writing portfolio.
To overgeneralize, you can generally get low double digit returns most years, while losing almost nothing in the bad years.
Or does that count as equities and therefore against the rules of the question?

Jim