Odd how the old memory plays tricks because as soon as I read your post I dived for the handbook convinced the +/- G values were in there. And indeed you are right because Publication CHK. P.N.-1 A.L.9 simply states:
"The maximum positive acceleration which the structure has been designed to withstand without permanent deformation at a weight of 2100lbs is 6.0g. Intentional manoueuvres must be confined to load factors well below this value."
The list of "approved" manoeuvres in the manual are all positive G - no negative G envelope is mentioned at all.
Again it may be fading memory but I also recall some documentation (CAA?) about civil Chipmunks suggesting they did not qualify for full aerobatic status i.e. +6/-3g and were only to be considered "semi aerobatic" which I think was to +3/-1.5. I'd prefer if someone could verify the "semi aerobatic" limits as I don't trust my grey cells these days.
There is a line in the handbook saying the aircraft is not designed for sustained inverted flight but that was not a G question - more to do with the engine (no inverted fuel capability and loss of oil flow).