merge master into the subtree branches

Hi everyone,

I have a large Git project which I would like to dissect into
subprojects with their own repositories. Git subtrees are ideal for
this task: I first

 * create a branch with the contents of only one subfoldergit subtree
split -P <name-of-folder> -b <name-of-new-branch>

and then

 * pull this branch into another repository.

For a transitional phase, I would like to have the subprojects
read-only and sync them from master. The question is how to organize
this. For every commit to master, I could of course perform the above
procedure repeatedly for all subprojects, but this seems less then
ideal since it does all the work all over again.

Is there a way to merge master into the subtree branches?

Cheers,
Nico