NARROW BAND
Yes you can plug a narrow band straight in the ECU and read BASIC AFR.
(Rich/Lean/Stoicometric) (Stoicometric means chemically perfect AFR that is not lean and not rich)
This is only good for idle and light load - ECU will always auto tune using
narrow band provided this basic mapping in these ranges is near enough.
On boost, the ecu checks for lean off conditions and will cut the engine if
detected but there is no guarantee that this will prevent meltdowns.

WIDEBAND
Wide band measures the correct AFR for all engine conditions.

I would recomend buying an independant wideband meter as you can leave this in
the car permanently to keep an eye on things (Confidence booster)
Plus you will be able to connect to the ecu to perform as per narrow band
and also use the autotune function (coming soon).
Also this will be able to check the actual AFR against the mapped AFR and
prevent meltdowns (Except if caused by detonation)


Hope this makes sense Phil.