My guess is it won't show B: either, if you had it.
Try something like
subst i: a:\, then see if the plugin displays I: or not.
Edit: Ok, so I see my guess was indeed correct, and the solution above works. As for why this is, well I'm no expert but we all know (or should know) how Windows treats A: and B: differently, so there's still a lot of software out there that expects to see long-term storage on the C: (and greater, so should those be called C++?) drives. No doubt this 'bug' affects the Explorer plugin for Notepad++ as well, which is why it doesn't display the A: and B: drives. SUBSTing either of these with another drive letter creates an alias that obviously the plugin has no problems displaying.
As for the problem of SUBSTed drives not persisting across reboots, there's a slight trick to accomplishing this. You can use the following command:
reg add "HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\DOS Devices" /v I: /t REG_SZ /d "\??\A:\" /f
to make the mapping persistent across reboots. Further details available in Wikipedia's article on SUBST, as well as this thread. You can also use the psubst batch file which makes it easy to create normal or persistent SUBSTed drives.