While I've only seen this ONE time. An SCA did offer to store a passenger's extra bags in the baggage car for them. He warned that they would not have access to it the rest of the trip. I watched him grab the bags and head off with them. No bag check in sight.
I wouldn't count on it but a heads up, client friendly SCA may do it if you ask and tip nicely.
This does happen sometimes. I've had an attendant do this for me on 448 (two or three times maybe), and on the Cardinal (once).
It used to be a bit more common, with some SCAs storing bags in vestibules and empty rooms and whatnot. Now that the baggage car provides the only space in most situations, fewer attendants offer to take bags.