Further info...
Just after posting, I noticed the ping to the printer was again failing.
I went to Internet Explorer and clicked on a few website links which all worked.
Then I tried the ping again and that worked.
I have noticed this once before... maybe an incredible fluke, but browsing to test if the wireless network is conneccted seems to immediately "fix" the problem.
Perhaps IE does something that wakes up the wireless connection, and the printer driver could be made to do the same trick?
I'll keep checking when I see it go offline and see if this is real.
I'll also try printing from the iPhone while in the broken state, then after watching that print job succeed, ping again and confirm the ping from Win8 is failing, then see if browsing brings it back to life.
Will post again if I can confirm a pattern here.