Making headway on this issue. Apparently just making a new file doesn't automatically work (not sure why) but modifying the nokia one in types/ did. So I just quickly copied the PC_FN_LEVEL2 button code as "PC_CTRL_LEVEL2", replaced LevelThree with Control, and tried it on the arrows. This HAS blocked the MicroB behavior of Ctrl-left/Ctrl-right being tab/shift-tab.

However, it's worth noting that, so far, this also seems to be blocking the more typical (and possibly desirable) behavior of Ctrl+Left and Ctrl+Right moving to the beginning/end of a single word, which I guess is a hildon-level thing or something.

EDIT: To clarify, to block that default behavior, you need to map it to something other than that arrow key, but then Ctrl+arrow will not behave like you'd expect (unless you expect it to act like a different key). Furthermore, if you map Ctrl+arrow to Home/End keys, instead of behaving properly, in MicroB it works like PageUp/PageDown keys instead (but worked right in Terminal and Notes). If you leave the value for the Ctrl+arrow mappings blank, then it completely ignores that keypress combination. I'll fiddle some more sometime, but that's what I've gotten so far.