I've added the K bootloader to the files section (and for fun, I dumped past it and found my arduino sketch right where I expected). This will let you restore a K board you've accidentally (or intentionally) wiped the loader on. I was only able to do this because someone sent me a K board (not naming them because I'm not sure if they want to be named). If you'd like to do the same on an S, I'll upload the sketch binary. You let it run, attach, then dump ram. You'll need to work out the ST-Link header pins - I don't know those for the S.

Edit to add - yes, this means I've run my own code on the K now.