So let me rephrase this; with the NC, you can boot to and run the OS and all it's apps off the SD card. But with the STR, you can't run the OS from the SD card.
With the STR, you can automatically run a script and/or some apps from the SD card at boot, but this is done by the OS in main memory.
So in order to have something like an n2a for the STR, you'd have to have two SD cards - one to load the open OS into main memory, and a separate "a2n" card that would re-load the standard OS into main memory. And since the standard OS is proprietary, BN would have to supply the "a2n" card.
Just found this: http://blog.the-ebook-reader.com/201...ket-and-gapps/
You CAN make your own "a2n" card by copying what's on the STR before you root it, but this is probably too much to expect from a normal user.