Sounds like an unacceptably high error rate. Perhaps best to stick to using it as a heart monitor, and find something more reliable for biometric login.
I've not done any research but I suspect that the pattern of veins etc in the rectum may be unique, so logging in securely requires nothing more complicated than sticking some sort of probe in to photograph it, and of course leave it inserted to prevent unauthorised access if the user wanders off.
One problem is that the veins and piles may well change over time, so a better option could be to look at vein patterns in the bladder. Just insert a fibreoptic camera via a catheter and you have the ultimate security system.