So I feel sheepish. Turns out I had installed the "release" apk from the main zidoo advert/download page and that is version 17.6. A newer Version 17.6.1 from GitHub as found via their support forum has a "Fix 23.976 framerate output". That is the only thing in its one line change log but I did notice the apk for the newer version is much smaller. After installing 17.6.1 the option 23.98 shows in the system settings now. Also want to give a shout out to the add-on "Backup" that allowed me to make a backup of my 17.6 zdmc and restore it after I updated to 17.6.1 (since it required a delete of the original 17.6). I mention just in case someone else figures out they have the older version. See post:
Here Worked great.
They really should really have the newer version on their release download page from their main site. Oh well. Search and ye shall find I guess.
Cool - glad it's sorted.
ZDMC has been deprecated and unsupported since they moved to HT2, so I guess that's why it needs a bit of sniffing out.