As far as I am concerned, this would be a very welcome addition to igraph’s C core. If you would like to give it a go, the first step is to get familiar with igraph and its quirks: compile the development version and make sure you can compile small programs that use igraph. If you need help, feel free to ask (I suggest opening a new thread in the Development category). I’m sure you’ll have questions. There are also useful resources in the Wiki. Once you got started with the basics, you can open a PR against the develop branch on GitHub and we can provide guidance there as you are proceeding with the implementation.
Implementing it in C is of course going to be quite a bit more work than doing it in R, so you should feel no obligation to do it. However, if you do decide to try, this would be a welcome addition.
As for incorporating an existing R implementation into the R interface, I can’t really comment on that as I am not deeply involved with that interface. We are a bit short on resources, and short on R developers. To be frank, the first priority is to simply update the R interface to use the latest C core, and get a release out. This is already taking up a lot of time. While I think it would be a good addition, I can’t personally make it happen, and I am not sure we have the resources right now do deal with it. Of course, it is possible that another maintainer would step up and be willing to incorporate the R implementation—however, there has no been any response so far. Therefore, for the moment I would suggest publishing the R implementation as a separate package that depends on igraph (if you are looking to publish it). Once again, this is just my opinion, and I am saying it because we are really short on resources right now.