You wrote:
- Is this about the time/mileage to expect the voltage regulator and/or brushes to fail?

It is about time that the Brushes are Worn down to nubs.

The very first thing to do is to remove and inspect the BRUSHES --- Odds are they are so worn down that they no longer make contact with the slip rings ---Thus NO Charging.

The Regulator/Brush unit can be removed without taking out the Alt. A Mirror will help you see the back of the alternator.
2 screws or Nutheaded screws(use a nut driver or 1/4in drive ratched) hold in the Regulator/brush pack.

Brushes can be replaced if you know how to solder and find the right size brushes.
Otherwise just buy the whole unit - regulator/brush.

You Also Wrote:
- Is there a way to coax a few more miles out of the alternator/voltage regulator?

NOT the Worn Down BRUSHES. The Voltage regular itself is probably OK.
IF you find that the Brushes are Long and not worn down then there cud be other problems, but the Voltage Regulator itself is way down on the list.

There shud be YouTubes showing how to do this on a Volvo 240 --- Try the IPD website. They have numerous How To Videos