Hi David W, With references to plastic in the bottom of your bean trench go for it, ;)
as long as you go down deep enough, (two spades deep) and puncher the plastic in a couple of places to allow excess water to run off, yes you can place in the trench any organic material (farn yard manure, garden compost, paper and cardboard, grass cuttings) my dad even used stinging nettles, :o said it gave the beans extra taste, :P but it no difference to a very large grow bag and we all use them, ;D i saw the same thing on reruns of Gardeners World on sky the other night, allotments around Birmingham & Manchester, he even earthed up his carrots and parsnips and the size was very good ;D
the only thing to remember is the digging rotation you have to get the plastic out (not sure if you can grow beans in the same place for consecutive years because of diseases) :-\
hope this helps
windy