The difficulty thing is getting a panel large enough to sustain the input to the charger to complete a full charge cycle. Too small of a panel and a cloud may drop the voltage low enough to reset a smart charger. Thats why a of solar battery chargers use just a simple trickle circuit (think cheap cordless phone where the batteries go bad after a year), or higher end have a battery in between.
If you do go solar charger I'd recommend then getting a home charger like the c9000 so you can test and refresh the batteries after the trip so see what damage the trickle charging did to them.
Eneloops take labels very well, a sharpie stays on them longer than other batteries I have, or maybe you can just see the faded sharpie against the white better, I don't know. But I number all mine as I buy them then mark a purchase date in a spreadsheet then run them through a refresh cycle on the c9000 and note the initial capacity. Then I'll refresh them once a year or so and note the capacity to watch for any that have dropped in capacity.