ah something i actually know a legit answer to!
the FJR and YZF master cylinders were taken out of the parts bin but use mirrors elsewhere, there is a yamaha plug that fits right in there and works great!
check out this thread:
http://www.vmaxforum.net/showthread.php?t=3026
i quoted the pics from an FJR and gave the part numbers, check out posts# 23 and 46
i think they're $2 or $3 apiece, expensive for a piece of plastic but fit perfectly. they're not threaded so they just plug into the holes
it also helps my old bike and my roommates up until a few days ago was a 2005 yzf600 so i just stole the plug out of it and made sure it worked first!