... Better would be copious digital photos of the plant, which are even lighter.  With current cameras and a decent chip, you have essentially unlimited photo capacity.
...
Thought # 2 - make good friends with a botanist.... 
Two helpful strategies, thanks for putting in thought time!
I do both limited foraging of plant parts and some digital photography.  However, upon reading the books I often find I missed something that is critcal for identification.  Often no big problem - I just return to the plant when I can.  However, I am in the field so the books are often with me in the field.
I have joined foraging and plant walk groups who include some plant savvy folks, including a botanist, retired mycology professors, etcetera.  I am cozying up to these folks as the opportunity presents itself, but do not want to be a pest.  Several folks are amenable to the email route as well.