I wanted the talk about clericals and vesture to cool down -- there are lots of other things far more important to talk about. But I also hate to see a discussion hang.
A few points in no particular order.
Butterick sells patterns for gowns: a pattern that doubles for the Geneva (preaching) gown and choir robes. (They also sell a cassock pattern, which I understand has become popular among Matrix fans.)
I confess I have vesture dreams of unrealistic grandeur. Once I heard about the epitome of High Reformed vesture: cassock and collar with bands, gown, hood, cape and cap. The sort of thing I imagine one would wear if accompanying Charles I to the block. But short of Jesus Christ returning to Earth via Siberia, I can't imagine an occasion that would require such a get-up. Still, I do have the cap.
Despite what Philocrites said earlier, and what is oft repeated: the customary black ministerial gown is originally a medieval ecclesiastic garment, and only later academic one, and thus not fundamentally one. Except, of course, that the universities came out of the church's need for trained professions. (Don't use a graduation gown in worship; they're cut a bit differently.)
But Philo is right to wear a gown -- even if occasionally, and I hope this addresses one of Matthew's earlier concerns -- because the gown (without stole, bands or anything associated with the ordained state) is an appropriate garment for anyone entrusted by the church to engaged in a ministry with a position of invested leadership in worship. (Is that vague enough?) For trust, read somewhere in the ambiguous "in care" process prior to ordination, or a locally appointed chaplain or lay leader.
It is ecclesiastic (pertaining to the church) not clerical (pertaining to the clergy). After all, if not directly "on the job" a gown would be an inappropriate garment for anyone to wear.
So, getting back to Chutney's point: clerical garb and not vesture is appropriate street ware. Mine is black suit, clerical shirt with collar or banded shirt with clerical vest and collar. But you have to be careful and thoughtful. Sacerdotal churches keep their priests in uniform 24/7. I only wear them whem I'm in a setting where I'm exercising a ministerial role. Wearing them is a public witness to the ministry, and there are many, many people who are glad to make you an object of projection. (And directions, as in "Fawder, do y'know the way to the Air and Space Museum?") If I lived in Boston, I might eschew them or at least out of doors. This is a fine point. Now that I'm not in a parish, I'd probably not now wear them to meetings (like I have done at General Assembly) but I do when I preach or lead a wedding. Sometimes if I have secular errands after a churchly event, I'll cast off the jacket and detachable collar. It can be a bit Johnny Cash, but that's better than the "Father Brown Goes to Safeway" bit.
Speaking of -- I think if you're not going to wear them in church, it is an abuse of your ministerial standing to wear them at a demonstration.
Comment to your heart's content!
Later. Chutney remarks that there's a parallel discussion at AKMA. Be sure to catch some of the comments; I would fit in well in that group.
Keeping with what Chutney also said, a former (Unitarian Universalist) minister of mine wore a white gown of a style normally used to baptize adults. (I think she had the weights removed.) And as I said before, I think I'd wear a white garment -- probably a surplice over cassock, specifically -- because of the white baptismal garment synchronicity. It also reminds me of that line in the Te Deum: "the white-robed army of martyrs praises you."
Now, not everyone needs a white robe or black for that matter, and perhaps I've come across as too percriptive. Read the foregoing as, "if you're going to do x, then do it well, and here's a range of traditional options." That said, Left Coast Unitarian asnwered his own question about what's proper liturgical attire at his lay-led fellowship: "In our fellowship, the custom is a suit for men who lead a service."