I am not a lawyer, but work part-time currently. I try very hard not to get sucked in, but some days are hard. I started putting my out of office on for email so that clients could forward urgent matters to someone else in my department (and I wouldn't have to be checking constantly to forward myself. I still find that I have 5 days worth of work most weeks, but only three days to accomplish everything. I like the schedule, so I won't complain since I know they really want me there full time but are accommodating me as a favor.
Ask for more telecommuting and greater flexibility in hours at the same pay instead.
This is often what I have seen too in my experience. I was part time for a while last year but I am now a government employee working in a non traditional role so ot was a little different. At any of my previous jobs when someone attempted it it was part time pay for full time work. The court schedule, client demands, and case deadlines didn't accommodate part time work. But it may depend on practice area too? I work in tax now and have worked in tax and probate in the past.
From a client's perspective, I think it would be very difficult to basically sign off one day a week. In my industry, when we need our attorneys, we basically need them immediately. We are having a lot of issues with our title company because they have one person there who does absolutely no work on Mondays or Wednesdays. It is very frustrating because we will need to basically work with two different groups of people so that we are covered on her days off. I've started going to someone else in her office first because I can't ever get anything done with her.
I don't know what your practice area is, or how time sensitive or demanding your clients are, so it may work out for you and this might not be an issue. Unfortunately, it is difficult to deal with from a client perspective. It is unfortunate that there are not more options for part time work in the law profession, but I suppose that is the nature of the business.