Those days are (probably) gone: You can "ASK" for anything and everything; but that doesn't mean that you will get it, or that you will even get a polite response.
I would characterize the Market as NEUTRAL right now; urging our Buyers and Sellers to be reasonable; to accomplish the ultimate goal; buying the house!
Good luck and may God bless
You said, "An agent representing a seller told us this is not how it works in Marin and it would upset the seller if we asked them to split the escrow costs." First, take a look at the first four words in your sentence: "An agent representing a seller" which says it all. They have the best interests of the seller in mind, not yours. Second, everything is negotiable. Third, we are and have been for some time, in a buyer's market. If your offer is otherwise clean and acceptable, the seller--if they can afford to--may be okay with absorbing some of your escrow/closing fees.
You can write a contract any way but I have been doing it 18 years here and it is customary for the agents to write this way. In Southern California it is split 50/50 but not here.
In CAR Purchase Agreement, there is a paragraph designates Escrow and Title fee responsibilities; and 'TRADITIONALLY', buyers will pay for the escrow and owner's title insurance. Buyers get to choose their own escrow and title insurance provider.
However, everything is negotiable; you can always try to negotiate that if the property is a true 'buyer's' property (depending on the specific property, location or price, a house may or may not be buyer's market'. Otherwise, buyers customarily pay for both in Marin county.
Frank Howard Allen Realtors