I think if you feel the interview has gone well, asking if it is a possibility is a good approach, particularly if like hatwoman, you only want to do 1 day a week less.
I think applying for a f/t job when you are only available 20 hours or something and not saying until later on in the process or after you've had an job offer is likely to irritate people though. I would not be impressed. If there was a really good candidate who asked at a late stage whether it would be possible to reduce the hours a bit I might consider it depending on the job.
I think waiting until a late stage and then announcing that you will need a job share partner is cheeky. Applying for a fulltime job with a job share partner you have already found is good though, would definitely welcome that.
Otherwise I would state in your application that you would be looking for a job share.
Usually if the role is ft it is for a reason, there is at least perceived to be that amount of work to do. Requesting flexiblity/reduction in hours once you are doing the job and able to make suggestions about how this could be done is one thing, but if you are not in the job you won't know whether there are tasks being done unnecessarily/things which could be arranged differently etc
If you do know enough about the circumstances/job to know that it would work more flexibly, I would want to see some kind of proposal about how it might work with your application, to demonstrate that you have thought about all the issues involved and can address them. That would impress me.
sorry for ramble!