I believe it is possible to configure the NVRAM to have a one time boot to another startup disk, but I'm not sure how that is done. The two commands which I am aware are "bless" which has been deprecated and the newer command "systemsetup". "bless" is difficult to use and understand, but I believe it allows a one time boot option, but how to determine the Windows boot file to use is more problematic. I'm not sure if "systemsetup" has a way of configuring a one time boot option. Be careful with "bless" since you may end up with more options showing up in the Startup Disk System Preferences or when Option Booting which will likely be broken (I have no idea how to undo a "bless"ing).
Check out the rEFInd boot manager which provides a menu for selecting which OS you want to boot and can be configured to automatically select a default OS. This is the best option in my opinion.