"gst-inspect cacasink" tells me it claims to support 15-bit video:

Pad Templates:
  SINK template: 'sink'
    Availability: Always
    Capabilities:
      ...
      video/x-raw-rgb
                    bpp: 16
                  depth: 15
             endianness: 1234
               red_mask: 31744
             green_mask: 992
              blue_mask: 31
                  width: [ 1, 2147483647 ]
                 height: [ 1, 2147483647 ]
              framerate: [ 0/1, 2147483647/1 ]

However, when I send a video of that format to cacasink, it crashes:

$ gst-launch videotestsrc ! 'video/x-raw-rgb,bpp=(int)16,depth=(int)15,endianness=(int)1234,red_mask=(int)31744,green_mask=(int)992,blue_mask=(int)31,width=(int)256,height=(int)224,framerate=(fraction)60/1' ! cacasink
Setting pipeline to PAUSED ...
Pipeline is PREROLLING ...
Caught SIGSEGV accessing address 0x7abc5ac
Spinning.  Please run 'gdb gst-launch 28374' to continue debugging, Ctrl-C to quit, or Ctrl-\ to dump core.

Unfortunately, actually attaching gdb doesn't seem to achive much:

Reading symbols from /usr/bin/gst-launch...Reading symbols from /usr/lib/debug/usr/bin/gst-launch...done.
done.
Attaching to program: /usr/bin/gst-launch, process 28505
Reading symbols from /lib64/ld-linux-x86-64.so.2...Reading symbols from /usr/lib/debug/lib/ld-2.12.1.so...done.
done.
Loaded symbols for /lib64/ld-linux-x86-64.so.2
0x00007fca3ef1f203 in ?? ()
(gdb) bt

+ Trace 225383

I'm using Ubuntu 10.10 on x86_64.