This second post also addresses Christopher Potts' book, The Logic of Conventional Implicatures (OUP, 2005). In chapter 4, Potts discusses various kinds of supplements, including appositive relative clauses. A general feature of supplements, according to Potts, is that they must be strictly adjacent to what they modify (the example number is that from Potts, page 104):
(4.29a) *We spoke with Lance before the race, who is a famous cyclist, about the weather.
However, the adjacency requirement appears to me to be relaxed just when another supplement is embedded in the non-local one:
(1) We spoke with Lance before the race, who, as far as I could tell, was primed and ready to go.
(2)??We spoke with Lance before the race, who was primed and ready to go.
(3) Bob Scalpbender came by, who, as you know, endorses phrenology.
(4)??Bob Scalpbender came by, who endorses phrenology.
The non-local supplement also has to occur rightmost; adding "about the weather" to (1), as in (4.29a), renders the sentence unacceptable.
This relaxation of the adjacency requirement is a rather odd fact, one which I have no account of.