There no officially supported way provided by B&N. The NT has been out of production for so long that the odds of B&N providing such support aren't good, thought it'd be an undeniably cool thing to do. There are costs associated with doing so, though, so it's understandable that they haven't. You can either root the standard B&N firmware to add GoPS access, or use alternate firmware (e.g. Cyanogenmod) on your device.
I have a Nook tablet with 1.4.3 and it is not rooted. I was able to "install" google play. I say "install" becuase i can only get to google play with internet connection via a website. On my tablet i went to crunchyroll.com. I thought I was going to have to watch crunchyroll via a webpage becuase i could not find the app in the app store. When i got to the website it gave me the option to install the app. I clicked to install and was then given the option to install google play. Google play finished installing and then i went back to the website and clicked to install the crunchyroll app. This took me to google play and i had the option to install crunchy roll. I also had the option on the top left corner to go to the home page in google play. At this point i think that i am in google play (i need to mess around a little more to know for sure) What i do know is that i was able to search for games like Candy Crush and install it. The problem is that i dont have a link on my Nook tablet home page for either Candy crush nor google play. I can access candy crush when i get a notification that i have full lives or by going thru the crunchyroll website. When i get to where i believe is googleplay i am unable to create a book mark. This makes me believe that it is not a website and that it actually installed. I just need to figure out how to access google play without having to use the internet.