Now that you mention it i haven't tried the E-3B
My understanding thus far is that CoD tries to model each control the way it was on the real aircraft. For example, if it has two position flaps like the Spit a simple toggle switch will do, if it has manual flaps like the 109 you need separate up/down keys and keep pressing them for as long as you want them to deploy, if it has a flap motor with up/neutral/down positions like the Hurricane, the 110 and the 110 then each up keypress moves the lever up but the flaps are moving as long as the lever is not in the middle neutral position, etc.
In that sense, i expect that on aircraft that use a gate or detent on the throttle between normal and WEP power it will be possible to map different zones of control on your throttle axis to emulate that behaviour, but on aircraft that use a control that's separate from the throttle you will probably need to use a separate key command or stick button that's mapped specifically to it.
I haven't tested this too much in regards to WEP activation, but it works this way for every other control (eg, flaps and bomb bays) between aircraft that have different control mechanisms.
Hope it helps.