OK, I tried Mastiff's suggestion to adjust loadouts in FMB and it seems to work ie. the load outs carry over to online (ATAG Server #1).
Per his suggestion, I opened FMB, made a simple "mission", in this case just a Spitfire Mark 1a 100 octane over the English Channel. In Properties I designated it as the "player" aircraft, then still in Properties I clicked on the Load out button to bring up the load out menu for all 8 guns.
For testing purposes I chose different colour tracers for each of the 8 guns, along with the usual AP, deWilde, Ball VII, etc. Did a "save as", giving it a unique file name "Snappers brew" for quick identification. Hit OK. Did a quick test over the water; all guns were spewing out their tracers into the water. There is no provision to adjust horizontal/vertical convergence in FMB that I could see -- hitting Options/Plane/loadout when playing the mission within FMB doesn't allow you to do it.
Exited FMB, logged onto the ATAG Server. At Gravesend selected the Spitfire Mark 1a 100 octane and low and behold: Snappers brew was available as a loadout for this aircraft. A quick check verified the loadouts were exactly as I had assigned them in FMB. I could also now adjust convergences. I chose a wide dispersion from 1000 yards (outboard guns) down to 150 yards (inboard guns). No one was around
, so I did a quick check sitting on the field -- easy to see all eight guns throwing their multi coloured strings of tracer. Presumably the other loads in with them (AP, deWilde, ball #VII) were also going out. Adjusted the convergence down to a more reasonable dispersion and went hunting. Shot down a 109, then got shot down myself in a suicide run over Occupied Manston airfield.
What a pain. But it seems to work. Lots better than shooting 2 guns out of 8, with the rest being blanks!