[Orgmode] How to do this by MACRO? (feature request)

[zwz]

I want to insert a series of images in the org file.
The images are located in a directory, with names in such a pattern
"cat-N00000.png", where N is a number.

The direct and compact macro is something like this:
#+MACRO: cat-imgs /path/to/cat-${1}00000.png

Here the {} is used to separate the placeholder from the zeros, which in
fact is not supported in current version.

I know that I can use 
#+MACRO: cat-imgs2 /path/to/cat-$1.png

but it is a little tedious when I use it.
I also tried to define a null macro first, which was then used as a
separator separating the placeholder from the zeros. But it fails.
#+MACRO null
#+MACRO: cat-imgs /path/to/cat-$1{{{null}}}00000.png

I wonder if there is a feasible way.