I'm a consulting engineer - close enough! love those good ol' boys.....grrrr. oh my, I just looked what part of the country you're in - might be a tough road to go down in St. Louis - I started my career in KC - my those Missouri boys are set in their ways! And they sure don't understand why you aren't barefoot at home in the kitchen! ok, maybe I'm being a little harsh, I left KC in 1995 - maybe it's gotten better.
I have been part-time since my first was born (he turns 11 next month). I was 30 hours for the first 3 years. then I was 8-10 hours, more as needed. both of those were with the same firm in Denver, with headquarters in CA - so they were pretty flexible. just switched jobs in January (still in Denver) - now I'm about 15 hours per week. I also know a number of people who do engineering part-time; most at 24-32 hours per week.
I gave up on being a project manager about 2 years after going super part-time. I just wasn't in the office enough to argue/maintain resources (drafters , young engineers, etc) on my projects. however, at 30 hours/week it was very, very doable to be a PM - I was co-project manager as the lead firm on a 100 million dollar treatment plant with a 7-firm consulting team. and the client hired me (oh, sorry, the firm) again - so they must have thought it was working!
here's your arguments back if you want to fight for it:
you will be in the office no less than any other senior person. seriously - senior architects/engineers aren't in the office more than 20 hours per week anyway! so, where's their reliability? wouldn't a set, shorter schedule actually be easier for everyone?
clients are already used to calling your cell. if that is a problem then forward the office phone to your cell.
you are already uber-organized. meaning - you are perfectly capable of planning your work out over the next few days so that you dovetail into your resource work plan (i.e., drafters, etc.) I'm going to guess you already do this.
will everyone else want it? who knows? the firm that I was with when I started down this part-time road had flex time for everybody. we had people training for triathalons and major bike races. we had people who went part-time for a semester or two to tackle a tough graduate course. etc, etc - it wasn't just the moms. but guess what - EVERYBODY was happier and felt more appreciated because their individual needs were recognized and taken care of. it does require good communication skills from everyone involved!
EVERY project should ALWAYS be double-teamed anyway. uh, have they heard of client service? Long before I got married and had kids, I ALWAYS made sure there was at least one person on every project who knew as much as I did - you could get hit by a bus tomorrow! Plus, if you're indispensable, you can't move to a really cool project or get promoted!
Its worked very well for both firms. I'm willing to take my hours up/down so I can take on extra work when things get busy and then drop back down - less need for the lovely hire/fire that happens frequently in our industry.
I'm sure you're wondering...if I'm not doing PM work, what am I doing? Senior technical resource type stuff. I'm the go to for the young engineers. I do a lot of QC review. I take bits and pieces of projects that can be parceled out (I.e., a pump calc, water quality calcs, etc). basically I've acted as an extension for a couple of PMs so they can work 50 hours rather than the 60+ expected in our industry.
this is doable. but it's tough in a lot of firms in our industry. and you can't walk away - more than a year away and you have to start at the bottom again - goodness gracious - you'd think they could figure this out.
you have to be very, very good at saying "no" in a way that doesn't sound like you're asking for anything special. if you keep your workload at 30-32 hours, everyone should be well served - including clients. and you will need to stay flexible - I've always been available for meetings outside of my "normal" hours if it's really needed.
good luck.