Interesting question, how can we make up for a missing "-F".
I shall omit the obvious answer to simply search without a fare restriction and ignore the unwanted fare basis. (but seriously, why don't you?)
One less obvious way would be to only look for fares that start with other letters, like so
F ..a-|..b-|..c-|..e- (omitting a fare basis code starting with the letter "d", for instance). But I haven't tried this with 25 letters of the alphabet, which you would neeed if you don't know anything about that airline's fare code structure
And of course, maybe you should rather look into ExpertFlyer for that