I found an old e-machines mouse for free, so that's what I'm using for my spinner. I noticed on the solder side of the PCB where a third button would be, there was a ground, dead pin, solder connection. I connected the dead pin to the ground, and whamo, third button

(This goes to the upper right most pin of the IC (361061-1000, 9930W BN97K2) while button one goes to the bottom right, and the second button apears to goto the one from the top right)
Hopefuly more two button mice are like this and some of you guys find this useful.