I've got no clue. In the overall scheme of things, hacking a bike is very low on the priority list, but consider this - if a foreigner is caught breaking ANY law, even a minor one, the low end punishment is to not allow renewal of residence permit (accompanied by a reentry ban ranging from a few years to eternity. The option above this is a free ride to the nearest international airport where you'll have to buy a one-way international plane ticket.
I'd also like to point out that software designed to do something against regulations often comes with extra security issues (like trying to hijack confirmation messages from your bank, etc.). After all, who's going to file a report that begins with "I was using malware to steal free access to bikes and the malware did something malicious to me!" down at the local PSB. If some does this, they can finish writing their report about how they got some instant karma from inside their administrative detention holding cell.
Bike rentals are cheap. Is it really worth risking your ability to live here to save a few kuai? Also, if enough people do this, the company offering the Yellow Bikes will end up going out of business since it won't be able to pay for maintenance and for new bikes.