I posted a blog entry about how I used the STM32L1 RTC (a few years ago) as a tickless, ultra-low-power timer for my RTOS. http://www.indigresso.com/_blog/?p=181
The MSP432 has a similar problem. Only the RTC and Watchdogs run in LPM3 and 3.5, no other timers do, but you want to use those LPMs for timed sleep! There is a workaround, and it is actually very fast thanks to the really great ALU in the CM3 and CM4 devices. My code is also implemented in bulletproof production firmware that I've shipped to various industries 
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Yes, the code is for STM32L1, but I think you should be able to very easily port it to MSP432. If you wait long enough, I already will have ported it, but I noticed some forum activity on this very topic last week and I had to step in.
Happy Coding.