I don't have anything much to add other than that I'll be following along with interest. I make costumes and kilts for highland dancers, and we have a few male dancers in our region - PC jackets can be hard to come by and there's been some interest in seeing if I can make them.

My dance teacher has made her son's jackets for a few years, but instead of working from the folklore pattern, used a tux-with-tails jacket with the right collar and modified it (cutting it down, using the extra material from the tails for the epaulets and other embellishments). It definitely worked well as a lightweight alternative for dance - and looked just as nice on stage as a wool PC, but that solution wouldn't work so well for formal occasions!

I've ordered buttons from both Tartantown and Highland Express for use on highland dancers vests and jackets, and the quality is very nice. I wouldn't have concerns about using them on the pricier jacket for your client.