Not a complete boom re circulation, but on my Case 4420  sprayer the boom section on / off control switches are mounted on the boom close to where each section start is.  So maybe 5-10 gallons left in wet boom instead of the 50-60 gallons it take to charge the whole system.

 Not sure how you could do the wet boom from each end of the clean outs unless you could run the pressure lower then the diaphragm check valves spring pressure so it doesn't spray out of the nozzles.

   So I put a tee in supply line just before each switch so I can recirculate all the supply line hoses which are pressurized all the time pump is running. There is a small amount left in the wet boom that doesn't get recirculated. Just used 1/2 inch push lock type fittings and hose back to just another on/ off valve going back into tank. Has been working good for 7 years so far.

   The trick is to remember to shut off recirculate valve before I start spraying as the flow meter counts the volume flowing through recirculate system.