Here's a really bad from line that wreaks havoc with my (table-based) index
I realize that I could simply limit the length of the line, but this seems
like a MIME thing and thus something that might be in MHonArc's domain.
Yep, it's MIME. It is part RFC 1522. Currently, mhonarc does not
support RFC 1522. For now, there is probably no acceptible work-around
for your needs besides limiting the From string length.
P.S. I am curious about your table-based index, and how it looks.