Quote - I think you've understood the problem.

Perhaps not :(

I thought you had different versions of the base as separate pp2s, but it looks like you are using only one version, and applying a fit pose to make it fit the other figures. Still the same basic principal should apply. You need a moveable point that that you can parent the smart-props to, and this movable point can be a ghost prop parented to the base. As you are using a fit pose rather than separate versions of the base, you would just use the fit pose to to translate the ghost into the right position relative to the base. I'm assuming that the attachment (tube) is a smart-prop, loaded after the mask is in place.

Quote - so what should the origin of the geometry-less prop be? the M4 height or the V4 height?

I don't think it really matters as long as it is somewhere in the "ball park". If the attachments (and thus the ghost) only need translation to put them in the right place, then the origin of the ghost could be absolutely anywhere. If the attachment also needs rotation in order to fit, then having the origin close to where that rotation needs to occur makes things easier, but you can still giggle the the position to fit via a combination of rotation and translation, even if the origin is not in the optimal location. It's a little more fiddly if the origin is not in the optimal place, but you only need to set the position the ghost once (for each figure).

I should add that this is all just theory, I haven't actually tested this idea.