PHOBoS tips all worked great for me, except one of my additional demos is coming out the same as a pre-existing demo. It's possible that it's my wiring that is at fault however.

I would however propose a variation:

PHOBoS's number of the extra sound buttons, keeps the patch numbers the same as the SA-1, but it means the existing buttons on the SA-8 front panel are kind of in a funny order (0,1,2,3,6 with 4,5,7,8,9 somewhere else).  

Instead my suggestion (Yes I've tried this) is to keep the existing tone buttons labelled 1 to 5, and label the new buttons 6 to 0, then re-order the SA-1 patch list to match this.

In other words:

Tone 1 button stays as tone 1
Tone 2 button stays as tone 2
Tone 3 button stays as tone 3
Tone 4 button stays as tone 4
Tone 5 button stays as tone 5

New buttons:

Tone 6 is a push-to-make button connecting pins 26 and 15 on the OKI
Tone 7 is a push-to-make button connecting pins 25 and 11 on the OKI
Tone 8 is a push-to-make button connecting pins 25 and 13 on the OKI
Tone 9 is a push-to-make button connecting pins 25 and 14 on the OKI
Tone 0 is a push-to-make button connecting pins 25 and 15 on the OKI

Now the two digit tone numbers 11,12,13,14,15... upto 55 are unchanged from the descriptions on the original front panel, but the new buttons allow access to the additional sounds.

The patch list arranged to match this button numbering scheme is: