I got it!
Using two helpers, one to stay in the center of the stage, not updating pos x, so able to move off screen.
This first helper is generated by the player in statedef -2.
That helper generates another helper which has the Interactive stage elements in it.
The first helper is used to track the center of the world with this.
[State 0, PosSet]
type = PosSet
trigger1 = !time
x = 0
y = 0
persistent = 1
[State 0, VarSet]
type = VarSet
trigger1 = 1
fv = 10
value = screenpos x
The helper that is generated by this one, will check that value and do some calculations to adjust the position.
That helper has this in it.
[State 0, PosSet]
type = PosSet
trigger1 = var (1) = 1; This is checking what stage I'm on.
x = (Parent,fvar(10)*.75)-640*.75
y = -245
persistent = 1
I have mugen set to
GameWidth = 1280
GameHeight = 720