I've just done some of the necessary research...
It seems to be that Option 2 from Vince's list above isn't an option. My new understanding (as of today) is that the software inside the instrument cluster is totally irrelevant when it comes to speedo signals. All instrument clusters pick up a pulse-based speedo signal; earlier clusters use the gearbox as the source, whilst later clusters use the ABS unit as the source.
If you fit a gearbox without a speedo drive (such as the 6-speed MYP gearbox), the cluster must then be able to pick up its speed signal from the ABS unit. However, if the ABS unit doesn't have a speed signal output, you've got serious problems!
My current belief is that the earlier MK20 ABS unit doesn't have a pulse-based speedo output, which would explain why it's fitted in conjunction with a gearbox speedo drive. Based on this new understanding, the 6-speed upgrade method suitable for a given car is therefore determined by the generation of ABS unit fitted, not the generation of instrument cluster. If you've got a MK20 ABS unit, you must upgrade via a gearbox extension (such as the one fitted to my car:
http://www.a2oc.net/forum/showthread.php?21523). If you've got the later MK60 ABS unit, both upgrade methods are available to you (either MYP 'box or an extension of your existing 'box).
Option 1 from Vince's list above remains a possibility. If this is physically possible, then the MYP gearbox option becomes available to all.
Cheers,
Tom