Why this concern about OEI and the unusable fuel on the failed side?
If you are OEI, the good engine gets what it needs. You actually have better fuel endurance since only one engine is running. Once you get down to the flange, you still have sufficient fuel from the good side for your 20-minute reserve.
If you are OEI and you are already below the flange, you would logically be within landing distance from somewhere to go. If you are not, what were you doing there in the first place?
Boost pumps in the 139 are only there to provide the initial pressure for engine start. They are not actually necessary in flight. If you lose an engine, you have the very same condition as above. The only difference is in the case of a pump failure on the good engine side. Crossfeeding will give access to that "20-min" fuel on the opposite side. So, in the end, the two aircraft have the same fuel capability.