We see Mr. Mustache in the courtroom in 1840 several times. I've noted it in episodes 1153 and 1166, but I know it's shown more than that. What's interesting is that, as I believe I've noted in the Watching Project, there's a judge in the 1840 trial who bears a passing resemblance to Mr. Mustache. If I remember correctly, the first time that we see Mr. Mustache hanging in the courtroom, Judge Mustache isn't there, but he turns up later.
I can't help wondering if Mr. Mustache and Judge Mustache are related. If that's the case, however, then Judge Mustache ought to be a Collins relation, since, as MB noted, Mr. Mustache occupied a choice position in the Collinwood foyer for a while. And if Judge Mustache is a Collins relation, then he should recuse himself from the trial at which we see him. But Judge Mustache doesn't look like the sharpest pin in the cushion, so maybe he just didn't notice that a relative of his was involved in the proceedings.