Is it possible to override the SD Card write protection setting in software? I'm using the built-in SD card reader on a Macbook Pro (15", late 2011) running Yosemite, and it treats every SD card as write-protected regardless of the position of the physical switch on the card.

Apparently this is a fairly common problem. I've already tried all the solutions given in this question, with no success. The only thing left that I can think of to try is making physical repairs to the drive. Since this problem is likely caused by a malfunctioning sensor for a setting I don't care about, that seems like overkill if there is a way to do it in software.

Is there any known way to tell the drive or the OS to ignore the write-protection setting and treat a protected SD card as writeable?
No.  You cannot override the write protection status of an SD Card via software.

The read/write status is handled by the firmware of the SD card host controller not by the driver.  From Toshiba (manufacturer of these chips):


  Content protection can be realized by using the [controller] with
  firmware.




Inside the SD card reader are a series of pins that get connected if the write protection is enabled.  This tells the controller that write protection is enabled (Picture is not of the MacBook connectors, but the best one to illustrate how it works I could find online).



It is most likely that these pins are "stuck" together causing the controller to "see" the card as protected.  You can attempt to clean it out or as a better alternative, just purchase an inexpensive USB SD Card Reader.