I have extensive experience modifying pump and CO2 backpack sprayers for applying treatments to research plots which could easily be applied to small-scale crop-production or mixed planting conditions (GH, trellis, field). I mainly dealt with R&D Sprayer (Bellspray Inc). You could check out their site for ideas if you're not already familiar with them.
My research focus is integrated disease management for orchards in the northeastern US, apple scab is my main focus. There are several projects I'm currently involved in that you may be interested in, including the use of a predictive model (RIMpro) and Potassium Bicarbonate (KBC) for apple scab control. Vincent Philion at Irda in Saint-Bruno-de-Montarville is a great person to talk to regarding RIMpro and KBC. The University of Vermont also has several individuals who are Farmhack members and others who are focused on orchards (Terry Bradshaw for organics).