The multi-DPI play store has been around awhile so you probably have it anyway. Before I installed it, I had a similar issue and I found that I had to clear the cache of play store and play services then reboot after changing the DPI. Someone else had success after clearing the cache of similar apps. Not sure how helpful that would be in your case.
Well this is interesting...since the time of my last reply I dirty flashed the last DU update and Office Mobile is no longer showing up as compatible. I'll see if I can change that
edit: yeah, it is available for me after setting the DPI to 480 and clearing data/cache of play store and play services. For some reason, the multi-dpi gapps package I used before isn't allowing it to show at 420. Someone else will need to explain why