Doing a google search, I found a vest extenders that was a leather piece that works as a jacket button extender.

I looked at Keltoi Gaelic Clothing site..our own slohairt, and he has a piper jacket that has a chain front button link, listed on a jacket.. You might contact him and see if he can get that link for you.. Of the two I found... I prefer the look of the one on Keltoi's website..