That's kind of what I thought...
So I tried the second option I mentioned. I mixed freeze-dried fruit, sugar and cold water.
It worked pretty well, actually.
These are the amounts I used, but it was just one attempt and I eye-balled it.
5.9g - freeze-dried raspberries, crushed
10.8g - sugar
9.4g - cold water
I'm sure the proportions could be improved. But it was a very acceptable version of jam.
I'm going to start taking a little baggy of pre-mixed sugar and crushed freeze-dried fruit
on trips. And some peanut butter.
(Raspberry "jam" on toasted flatbread)