If I were to move forward with mastering in that room, this would be my plan:
Study up and treat the room as well as possible, knowing that no matter what you do there will be peaks and dips in the low end.
Experiment with speaker and furniture placement/orientation while shooting the room with pink noise/tones and an RTA.
Try unorthodox placements that aren't "supposed to" work. You might be surprised. Maybe.
Try using multiple sets of monitors with different placements that complement each other's nonlinearities.
Make note of where the problem areas are, and especially how they differ when you move yourself around in the room.
Move yourself around in the room when working.
Supplement your monitoring with some great headphones that are flat through the lows. You won't "feel the bass" with phones, but they can be a useful check in a questionable monitoring environment.
Use reference tracks and spend a lot of time learning the room.
Check masters in other locations until you learn how translation will be.
The whole thing will require extra time, effort and expense, but in the end you can likely make a go of it!
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