Embedding a github repository inside a mercurial (kiln) repository - how integrated is it?

Question

Summarised Question:

Are github-hosted sub repositories within a mercurial/kiln repository possible, and if so are they automatically updated/cloned when the parent mercurial repository is operated on by a hg clone or hg commit command?

Detailed Question:

Following on from my question that was answered so excellently here , some of my third party code is in folders I downloaded a while ago from opensource efforts on github. Since at that stage I was not using version control, those folders where just standard folders that now been incorporated as sub repositories in mercurial.

This is obviously not ideal, as for one thing, new versions of the libraries may have bug fixes, or new features I wish to use in the future. I also may need to locally customise some of the libraries.

I can see from reading this link that it possible to have mercurial "know" about those git server urls (and revisions), so I can then have mercurial clone the github hosted libraries direct from their parent repos.

Am I right in saying that when I clone the parent (mercurial) repos, those files will be pulled from github, without having to separately manage this using git?

What is also not clear is, if I were to do this, and it transpired that code might need to be customized from within that github-cloned repository, would I need to use git to manage revisions of the local files, or would mercurial do that by proxy? eg id I were to hg commit -S would mercurial invoke git on my behalf to handle that?