url.<base>.insteadOf vs. submodules

Hi all,

I am trying to make url.<base>.insteadOf work on the URLs inside .gitmodules, but it won't work (applying it to the repo itself works fine, to the config setting seems to be fine).

I do not want to modify .gitmodules: It is maintained upstream.

I cannot simply reconfigure submodule.<module>.url: the Configure script 
(regularly called during each compile) does
   git submodule sync
   git submodule update --init
I could tell upstream to change these commands if I can make a good 
argument; for them, it is relevant that they can change the submodule 
URL inside .gitmodule and have it "just work" for everybody downstream.

My own use case is that I want to be able to work with various experimental local clones even if I do not have Internet access. I'm all ears if there's a way to do this without using insteadOf.

Here are the relevant two lines from the output of "git config -l" (after "git submodule init"):

url./home/jo/Projekte/perl6/bare-repos.insteadof=https://github.com submodule.3rdparty/dynasm.url=https://github.com/MoarVM/dynasm.git

Here is what "git submodule update" does:

Cloning into '3rdparty/dyncall'... fatal: unable to access 'https://github.com/MoarVM/dyncall.git/': Could not resolve host: github.com fatal: clone of 'https://github.com/MoarVM/dyncall.git' into submodule path '3rdparty/dyncall' failed

Any help appreciated!

Regards, Jo