I mentioned this 55 pages ago -- I stumbled across a bunch of LIRR employees on the way west through Hillside Facility one very early morning. I asked them about the full service Belmont from both directions -- and their general answer was not going to happen your lifetime.
I'll suspect that the better way to manage would be to run extra local trains westbound on game days/times which will pass Belmont, stop at Jamaica, and then head back east into the station. "Local train to Belmont, stopping at Central Islip, Brentwood, Deer Park, Farmingdale, Bethpage, Hicksville, Mineola, Jamaica and Belmont. Change at Jamaica for all other stations. Lets go Islanders/"
Inconvenient and a time waster, but its far easier to deal with than having to change trains entirely. And other than revamping the existing Belmont station to accommodate extra volume and trains , it wouldn't require the massive track work for the first scenario.