The attached patch what works for me.  It is however the result
of a short trail'n'error session, since I'm by no means an
experienced lisp/elisp programmer.

With the patch the behavior is slightly different than usual with
lists.  Before TAB cycles the list, the cursor jumps out of the
Latex fragment:

action:
 - $y = a_0 + a_{1[TAB]}$
  + foo

result:
 - $y = a_0 + a_1[CURSOR]$
  + foo

action:
 - $y = a_0 + a_1[TAB]$
  + foo

result:
 - $y = a_0 + a_1$[CURSOR]
  + foo

action:
 - $y = a_0 + a_1$[TAB]
  + foo

result: emacs mumbles "SUBTREE".

action:
 - $y = a_0 + a_1$[TAB]
  + foo

result:
 - $y = a_0 + a_1$...