Not a mixed class fare, but I recently booked FCO-LHR-SIN, SIN-LHR, LGW-TRN in business.
The longhauls are in I and the shorthauls were in the J fare bucket, which surprised me when I booked it (but the price was still good).
Expertflyer is showing FCO-LHR as J9 C9 D4 R9 I9 and LGW-TRN as J9 C8 D4 R9 I8 so there's plenty of availability in I, wasn't sure why it went the way it did.