You can send your gun out and have it sent right back to you, so FFL fees aren't involved. I have found however that if you have a reasonable FFL he can ship it a LOT cheaper than you can. He can use the USPS and you can't, you can only use FedEx overnight to the tune of about $60 with insurance.
To get to your question, any gunsmith you send it to for the metal work can then forward it on to the plater and then send it back to you, all he has to do is drop it in the mail. I would VERY strongly recommend you have the gunsmith send it out to be plated at a shop he recommends, otherwise he will not have the chance to check it over after it is done and probably won't stand behind anything that was done to the gun since you sent it out and had it plated on your own. If HE sends it out for plating and gets it back before it goes to you he can check it over, and HE is the one that outsourced the plating, therefor he is on the hook for everything that was done.
I would recommend that you pick your smith, send it to him with a VERY specific list of what you want done that confirms everything you have been discussing including prices and lead times, and leave the rest up to him.
EDIT: Plating, especially hard chrome, is forever. It makes the gun VERY difficult to work in the future. Have ALL of the work you will ever want to do completed before plating, if you don't you'll have a hard time finding anyone that will even touch it and a harder time paying for it with the added costs.