<div class="r-stack>
  <img data-fragment="fade-out" src="...."/>
  <img data-fragment="fade-in" src="..."/>
</div>

The derived backend (org-re-reveal) already has an 

#+ATTR_REVEAL that an make the data-fragment attributes, so it's not hard to produce
 the desired outpu:

#+begin_r-stack
#+ATTR_REVEAL: :frag appear
[[imglink1]]
#+ATTR_REVEAL: :frag appear
[[imglink2]]
#+end_r-stack

However, I'd really like to add a less verbose syntax, like this:

#+begin_r-stack :frag (appear appear)
[[imglink1]]
[[imglink2]]
#+end_r-stack

My question is: will the exporter preserve information from these header-like arguments, and is 
there a mechanism I can use in a custom ~special-block-function~ to make use of htem?

Thanks for your help as always!

Matt