You could go open loop and just check youself with the wideband that your system's doing what you want it to. You don't have to feed the wideband signal into the ECU. It's well worth having as an independent check IMO.
it is possible to wire in a lambda to the digifant ii ecu, the haynes manual actually has the lambda on the wiring diagram, and is usefull for a change! I'm not sure what kind of probe you need, I'd assume a 3 wire heated jobber like most other VWs of the same era.
it might be useful to have a lambda sensor fitted to a gauge to check the condition of the exhaust gas. even if its not part of the efi control. as Mark says. its probably a luxury in reality but could be handy for setting up the fuel map out on the road.
