You could modify the source a bit and compile yourself a customized bootloader that uses a different button to boot the OF; that's one method.
There's another method as well: if you're talking about just the plastic hold button being broken off and not the switch itself (which is part of the mainboard), you can take something small like a spudger or even a small tweezer and carefully flick the hold switch mechanism to boot into the OF.
And while this may be delving off the topic a wee bit, you can always replace the hold switch, which is part of the entire top bezel of the nano, with
this part.