You know what...I really think you can pull it off. As you say, it is a very dear friend of yours and you know best how he/she would feel about your appearance. And from other threads (recently) we know you have a distinct personal style--and that's NOT code for something sly or underhanded. Like you say, the spats are ummm....non-negotiable.

I think the colors will compliment well with the Navy Edzell tartan. I'm not so sure about the shirt, it seems a bit fluffy to go under such a well-fitted (looking) vest.

In this case, I think it really is about how comfortable you will be in the vest.

I say go for it! But, then of course, POST PICTURES TOO!!!