Back to ITA again; this may help you.

in the FROM / TO sections use ITA routing langauge

as an example

LHR:: lh+ /f bc=W
FRA:: lh+ /f bc=W

Says find Heathrow to Frankfurt, only on Lufhansa metal, only booked in 'W'. This only works for specific dates, not for month long search. You'll still need to get your travel agaent to book for you and you may find it easier to use seat counter to establish if the booking class you want is available for the dates you wan to fly.