Can a Mac boot when the right shift key is broken in the pressed position?
Sadly, you might need to open the Mac and disconnect the keyboard.

Here is an image from iFixit.com's takeapart for the 15 inch model.



It is protected by a metal cage - but you won't need to do all the steps - just get the bottom off and disconnect the battery before working on the keyboard cable.

You will of course need a USB keyboard to work things temporarily and enable the bluetooth keyboard if you can't get there with a mouse or the trackpad only. Do read the cautions on the repair guide and be sure you are up to a delicate task. A proper Mac technician might charge $30 for this repair since it's quite easy for someone with all the tools and experience getting in to the Mac.