Meals aboard JR trains?

In order to maximize the sightseeing time, we'll be travelling often on the first morning train (e.g. Kyoto - Hiroshima, first direct Hikari train 7am-9am, or Takayama - Tokyo dep ~6am first set of connections that works out) or last evening train (e.g. Himeji - Kanazawa, one of the last evening connections that works out) - that is, during breakfeast or dinner time. Also sometimes it may save time to eat lunch on the train (e.g. Hiroshima-Himeji).

I've read that JR does have some food for sale on the longer trains.

What - if anything - should we expect to be available on board for sale in terms of food? And on what trains (generally) would it be available, and on which ones not?

That is, should we plan to get to the stations early and buy some bento box or such to take on board prior to departure (surely should be lots of choice in/around the station for such), or can we count on something being available on the trains?

What - if anything - should we expect to be available on board for sale in terms of food?

A very limited selection of what is available at the stations, e.g. maybe one or two types of bento boxes, sandwiches, snacks and drinks. So, I strongly recommend to purchase your meals at the station before boarding rather than purchasing it on board. Stations offer a much larger variety of ekiben ("station bento boxes") with various ingredients. Often there is also a bakery with various tasty breads. As well as convenience stores with another range of food and drinks.

And on what trains (generally) would it be available, and on which ones not?

It is generally available on shinkansen and limited express trains. But there are exceptions.

No bento is available on the early morning long distance trains. Expensive Coffee & some snacks only until they get the fresh delivery & load up at some station along the way. Also bento kiosks are still closed on the platform. Some tiny snack/drink/newspaper kiosk may open around 7am. Some fast noodle shop may open early, too. I usually buy something at a convenient store the night before or on the way to the station & a hot coffee/tea can from a vending machine while waiting on the platform so I can have bf on the train.

Normally I would buy the lunch boxes at the Ekiben on the platforms, but a lot of them are closed if you want to take an early train out and we do as amazinga suggested, we pick up some sandwiches or so the evening before and hope for the best to be able to buy something on the train.
The choice in the train is often limited, but not bad at all. However, it is like choosing blind as all boxes are wrapped and it is not always easy to understand what's in them. I therefore prefer to take a little time and pick my bentou at the Ekiben if possible: http://hoshisato.aminus3.com/image/2010-03-13.html

I agree with the others about trying to buy the various eki-ben that local train-travellers crave for. But If saving time is what you are after, I recommend you to sleep in trains when meals aren't available at the stations. Quite often you can get better food at sightseeing spots where all the venders are. Also, during the day you can also find some hot soup-y meals on platforms like the kishimen noodles on Nagoya shinkansen platforms. You can't enjoy these things inside trains.

Visit here for all available ekibens.
http://www.ekiben.or.jp/main/
They show local & seasonal gastronomical flavors that you can not get at a local cookie cutter menu restaurants. And it is a small basic cheap kaiseki.