I emailed the contact address on the Changi website and asked if, similar to the US, whether a passenger could opt out of the body scanner. I did not say anything about medical conditions.
They replied:
Dear Zorak,
Thank you for your email.
Should you wish to opt out of using the bodyscanner at security screening, please inform the security officer on duty. You will be directed to our Walk-Thru-Metal Detector followed by a pat down after.
Yours sincerely
A.....Y.....
Senior Manager Avsec
Changi Airport Group (Singapore) Pte Ltd