I've just been looking at average freight per flight on ULH routes (from US BTS T-100 data) for 2017 and not surprisingly it is not too high westbound and the forward hold will likely be empty:
UA LAX - SIN (789) 2.7 tonnes, average flight duration 17:07
AA DFW - HKG (77W) 2.3 tonnes, average flight duration 16:04
The results will obviously vary by season and indeed day depending upon the jetstream, route taken, holding, etc. However consider the eastbound route:
UA SIN - LAX 9.4 tonnes, avg flt dur 14:34
AA HKG - DFW 12.3 tonnes, avg flt dur 13:54
So you may well want to use the forward hold in one direction. Also what if the aircraft finds itself on a west coast to Tokyo rotation? You could dedicate an aircraft to ULH routes but that would reduce flexibility. I believe that Pan Am (and doubtless other operators) switched 727 QC between passenger & cargo configuration in a matter of hours but doubtless there was a weight penalty which negates one of the reasons d'etre for deactivating it.
My solution... It's already been discussed in another thread, but I believe that there is a lot of scope for utilising the forward hold with suites, toilets, galleys, etc releasing a lot of space for pax, although of course it will add weight.