Hmm, later boards have a diode from Reset to 5V also to prevent spikes on the Reset line from fooling the chip into starting HV Programming mode, and then looking hung.
But - a Nano does not! So, as chrisknightley points out, the 1K pullup resistor would allow (Nano V3 anyway) could allow damaging current into the 5V line and fry other things. And that 1K is part of a resistor pack, so you would have cut a trace to remove RP1D from the Reset Line to let 12V be applied without damaging other bits of the board.
An external 1K or 10Kk could then be added to the Nano to allow normal Reset operations for ICSP re-bootloading or serial downloads.