Using the associated template in an external command

I would like to define a mapping in vim that looks for a special place in the file (using a regular expression) and executes an external command that depends on the matched pattern. That is, I am thinking of something like:

:g/set\ "\(.*\)"/!echo 'bla '\1' blubb' > file

or

:g:set\ "\(.*\)":call system("echo 'bla ". submatch(0) ." blubb' > file")

My goal here is to look for a line like

set "x=3"

and create a new file with the contents:

bla x=3 blubb

Unfortunately, both attempts (and several others) do not work. Thank!

Edit: adding the following to my .vimrc did not help either:

:map <F4> :g:set\ "\(.*\)":call Gnutex(submatch(1))
function Gnutex(str)
   return system("echo 'bla ". a:str ." blubb' > file")
endfunction

Edit: Working solution (along with the function from earlier):

:% s/set\ output\ "\(.*\)"/\=[submatch(0), Gnutex(submatch(1))][0]/

I think there are better solutions, but it worked.