I would like to make a booking for later this year on a popular route where I class is non-existent.

I have a GUF1 available and reading through the voucher conditions on BA.com there's no mention of specific fare class availability being required, but obviously it's detailed in the Wiki here.

So question - can a Joker be used with a GUF1 on a flight with zero availability in I class in the same way that a Joker could be used to force reward availability to use Avios instead? I thought it was worth a call to ask.