Do you actually have any media outside of 576@25 (PAL) and [email protected] (NTSC) .. because ye olde analogue broadcast standards certainly don't include those refresh rates; which is prob. why Linux doesn't invent/expose them.
480p23.976 isn't that unusual. If you have movies or TV shows shot at 23.976fps on film, and carried as 3:2 29.97fps interlaced on DVD or in MPEG2, some people will transcode the MPEG2 to h.264/5 480p23.976 to allow the content to replay natively at 23.976fps avoiding 3:2 pull-down judder (or to avoid their TV having to do 3:2 detection and removal).
Personally I'm fine with this stuff replayed at 1080p23.976 as I doubt my TV's upscaling can make much more of a silk purse from a 480p sow's ear...
(I don't think 480p23.976 is a CEA supported HDMI mode anyway)
However in 50Hz land, 576i25 (aka 576/50i) is the most common format - though in Australia they flirted with 576p50 as an 'HDTV' standard for a while... (Long since ditched for 720p50 and/or 1080i25 now)