Assuming a hypothetical `(pcase-assert OBJ PAT)' I would expect the
following to succeed:

  (pcase-assert '`,nil ``,nil)
  (pcase-assert '`,1 ``,,(pred integerp))
  (pcase-assert '`,@(list) ``,@,`(list . ,_))

which is to say: a comma should match its argument as a pattern only if
it escapes the original backquote (currently comma at all depths
"escape" the original backquote).

In case there is any interest in this, I have prepared a patch which
both adds the above as tests to pcase-tests.el and implements them in
pcase.el.