Wbar can't do it, it's a window manager setting. The people at Puppy forums thought about this too, I believe they decided creating a 1x1 pixel tray to the right part would limit window maximizing correctly - leave space for wbar. Add something like this to .jwmrc-tray (outside other Tray tags):
<Tray x="500" y="-20" border="0"></Tray>
You might need to adjust the y to make it look better.