Any idea what the thread size is that retains the valve?
You can obtain screw fit schraeder's as per the pic.
The larger thread measures at 10.1 (SS) & 10.3mm (ALU) OD but same thread.
If in UK could post you an aluminium one if it looks likely - would need cutting down etc.
You could be onto something here.... possibly the rear shock is goosed and these are actually gas struts - no luggage is ever carried, they just needed additional mounting points!