How do I detect the Computer's operating system?
If you can't get the OS from the USB port, why not use the existing keys/inputs to set the mode on startup? That is, if you tell the user to press different combinations of 2 buttons on startup, you have 4 (2 bits => 4 values) different paths on startup:

00 =&gt; Normal startup, don't change mode
01 =&gt; OS 1
10 =&gt; OS 2
11 =&gt; OS 3