[snip]

So what does it do when descending hills, assuming you approach the
top of one with a fully charged battery? Two questions really:

Does it 'coast' if you don't touch the brake like a car in neutral or
'hang' on the motor / transmission like one in gear but with you foot
off the throttle?

[snip]

The high voltage battery can never be fully charged under normal driving.
It is only charged to 80% capacity and allowed to discharge to 40%, this
preserves the cells.
The energy monitor inside the car normally has one bar left.

So when descending a hill with a fully charged i.e. 80% battery, the
regenerative braking charges the battery to 100% filling the remaining bar
on the energy monitor regardless of the gearbox normal or the additional
braking mode.

It takes quite a bit to fill this one bar and I have only seen it happen
once in the two years I have driven my Prius generation 3.

The transmission never coasts it is always engaged going down a hill is
like a manual car with your foot off the throttle.
I hope this answers your questions.
--
John Bryan