I dont do trenches as such, since i grow my beans in fixed beds, which usualy have something growing in them over the winter - so no space for a proper long term trench of the type we are talking about.
However as soon as I have the bed for that years beans clear, i dig out a narrow but deep trench and partly fill it with some form of organic matter, usualy rough compost from the heap/bin, but recently i have used shredded paper which is wetted before the trench is backfilled and the soil firmed in readiness for planting out.
Although the last 2 summers havent been the best for beans, this method seems to have worked well. it doesn't seem to matter either if the paper is still quite 'fresh' when the beans are put in, as long as it is wetted thoroughly before backfilling it seems to rot down quite quickly and not harm the beans. That said the earlier it is done the better, as it will have rotted down more before planting.
As for the traditional method, i dont see anything wrong with it as long as you follow the same rules as for proper composting, ie no weed roots or seed, nothing diseased or too woody and you mix the different material up well when you put it in the trench, then you should be fine. It probably wouldnt hurt to put a layer of soil on between layers, particularly if the trench isnt going to be added to for a while. Would stop it either drying out and/or smelling and would discourage vermin. I guess it wouldnt have to be soil, anything to cover it would suffice.
Hope this helps
