If an agent who gives editorial feedback offers representation, is it appropriate (as part of the pre-acceptance process) to ask them to pick a section of the ms they think needs work and give editorial remarks? That is, in addition to discussing their vision for the ms in a general way, which I seem to recall you saying a writer should expect. The purpose being to see what it would be like to work with them on that level. If that's okay, rather than epic ass-hattery, what size section would be appropriate and not an imposition?
An agent is not auditioning for the lead in Editor On the Roof.
Either she's already given you notes; she thinks chapter seven is fine; OR you're going to get notes and see what she thinks. It's entirely acceptable to ask "do you think the manuscript needs work; if so, what kind, how much etc." You can even ask "do you think chapter seven needs work?"
BUT saying "can you give me edit notes on chapter seven before I decide to say yes or no to your offer of representation" is not something you want to do.
Presumably by the time you've gotten to this point with an agent you know something about how she works. For example, you might have asked her clients if she gives notes to them, how fast that happens, how useful they are, and so on.
You might have read her blog to see if she can string words together herself.
You might have actually done a round of revisions with her before the offer was made.
But asking someone to give notes as part of the offer fails to recognize that until she sells something, she's got no income from you. In other words, you're asking for a favor. You'll want to be judicious in those asks.
You can get this information, or enough of it to be useful for your decision, without asking her to do more work than she has offered up voluntarily.
I'm glad you asked this question because it is one that would make me reconsider taking someone on, and I'd hate to see that happen if the writer just didn't know how it would be perceived on this end.