Modern mini-split heat pumps are inverter driven and can step up/down their power consumption and cooling capacity pretty much in 1% steps — at least that’s what the marketing material claims.

Normally the cooling output is a function of the set point temperature vs current ambient and the fan speed.

So I’m not sure what you would need to add to allow the units to operate/respond to variation in available power from the panels but certainly they are able to operate at variable output levels.

Is that not exactly what a brownout would cause, as GP alluded to? (I don't know the effect of low voltage on A/C so I'm guessing here...)

I don't think a brownout would cause the desired effect (GP said "brownout and fail"). But what you could do is automatically switch off a selected set of A/C units when power output is low. That way you can prevent a brownout (drop in voltage), while still having some cooling. Of course, the A/C units should be designed such that frequent shut-offs are not a problem.

Anyway, I'm no expert either but I think the approach should consider the whole system (PV + A/C) rather than view them as different domains.