My thoughts...

I regularly wear a kilt to church, and our congregation is small enough that after a couple of Sundays, I think they now expect it.

Our church is similar in dress... With a few older gentlemen and the pastor wearing suits... (I always wore a suit and tie, despite being the youngest in the group at 42), so I almost always wear a tie and argyle jacket.

This Sunday was different as I had to leave a little early for another engagement. I treated it like business casual (nice gray dress shirt NO tie or jacket), simple brown shoes and a brown day sporran. Gray hose with gold garters. Oh, I was wearing Irish National tartan.

My recommendations are similar to tobus, my only deviation would be a light gray dress shirt (add it to your collection for work... ) it adds a fresh element without bringing too much color and a level of formality without having to sport a tie. I think the claret hose and oxfords and your semi-dress sporran would be smashing!

Good luck and post some photos!