(I need to use OSMC for an upcoming video night, and enticed by reports from another user - Infrequent Random Crashes When Playing Video - #17 by megardi - I first tried to disable ‘omxplayer acceleration’ before going full rollback.)
Setup: as in the top post, except that ‘omxplayer acceleration’ was disabled, the RPi rebooted by power cycling, and left to its own devices for for ~5 hours. When I checked in on it, it had frozen up, as described above, even though the system had not been touched at all. 
Following a hard reboot, ssh and ‘grab-logs --all --copy’, the relevant log entry is ‘Kodi Old Log’: http://paste.osmc.io/ugelagewad.vhdl
Anyway, commencing with downgrade to the release ‘OSMC_TGT_rbp1_20150628.img’ in the hope that video night goes well and without hiccups.
Update: Upon trying to write the Agust disk image to the SD card, Win32DiskImager failed with the message that my 4GB card was only ~240 MB. I tried various ways of formatting it, but failed to re-gain the full 4 GB and started to suspect I had a Chinese knock-off. However, this guide worked in properly reformatting the card: http://www.instructables.com/id/How-to-format-your-SD-card-back-to-the-original-si/step2/ADVANCED-PROCESS-DiskPart/. I’ve recording my steps here in case others run into problems while rolling back:
- command line: diskpart
- list disk; select disk [#number-of-my-SD-card]
- clean; (pop card out, then re-insert); list disk (to confirm proper size)
- create partition primary; format quick; exit
I then wrote the disk image ‘OSMC_TGT_rbp1_20150628.img’ to the card and booted into OSMC.
Update 2: My SD card apparently got messed up while ‘clean’-ing it with diskpart. After the cleaning, I couldn’t ‘list disk’ or even restart diskpart anymore, not even after rebooting the PC. Windows couldn’t format the card, but the SD Association’s formatting tool was able to quick format the card (SD Memory Card Formatter | SD Association), after which I could write the disk image as usual.