What about additions to all the WW1 ranges for infantry and LMG figures in prone firing positions and just head and shoulders firing to repressent troops in trenches and shell holes? Someone does this sort of thing in 10mm WW2 but only in sandbagged fox holes which aren't right for WW1.
Your better off with normal figures in purpose built trenches, WWI trenches were quite wide and got wider in 17-18 for the Germans to stop tanks. The idea sounds good, but it's not practicle.
IanS