If you want a shuttle between Junction and Pen Mill the obvious place to platform it at Junction is the old cattle dock opposite platform one. Would avoid conflict with the two current working platforms (though would require use of the footbridge to transfer, though everyone entering or leaving has to use that anyway).
Would need a thorough rebuild/resurface to bring it up to spec.
The assumption is you'd use something cheap and lightweight, like an update of Parry's designs (what was that lightweight unit the French announced recently?)
Pen Mill would be more of a problem. You'd have to add a new short platform on the easternmost through line and extend the bridge to it. Prohibitive on it's own, but if the closed Pittards site were to be purchased and repurposed as car parking it could all be done as part of the same project with the footbridge extended into the new car park