The OnActivate event will detect activation though, the result of the keypress, in which case you'll already be in the process of dismounting.

Perhaps:
Int iActivationKeyScriptName MrEdScript Extends ReferenceAliasEvent OnActivate(ObjectReference akActionRef) iActivationKey = Input.GetMappedKey("Activate") Input.HoldKey(iActivationKey) RegisterForKey(iActivationKey)EndEventEvent OnKeyDown(Int aiKeyCode) Debug.Trace("Do other stuff, but don't dismount") ReleaseKey(iActivationKey) UnregisterForKey(iActivationKey)EndEvent
Of course, that does require SKSE, but something like that might pull off the dismount intervention.
Oh, try http://www.creationkit.com/BlockActivation_-_ObjectReference on Mr. Ed?