[SiriusHardware]

You could maybe panel-mount a 15 way sub-D connector in a cutout on the right hand side of the enclosure and wire the external keypad connector to that. That way you don't have to have anything non-period inside the enclosure, you just plug your uploader of choice into the sub-D connector, load something up and then unplug it.

It would mean modifying the 'historic' housing slightly, but sub-D connectors are as 1970s as they are current, so it would look perfectly in keeping. It would also let you plug in a high quality external keypad or even a keyboard with more keys on it, should the mood ever take you. (The MK14's row / column matrix can support quite a few more than the 20 keys on the original keypad).

Edit: A full sized keyboard would need to use all of the row / column lines including two which the standard keypad does not use, so if that was the plan it would be better to go up to a 25-way connector (there are 19 way sub-Ds but they are rather rare now).