I've also removed the dash without removing the windshield. taped the top of the dash so not to scratch it and held the flap of the seal up with a 5-in-1 while using a 1/4 inch wrench. I installed the screws using a lot of patience and tweezers with butyl (dum-dum) on the tips. Time consuming yes, but do-able
I've got a very old tech article with pictures on removing the dash in a Maverick with the windshield still in on my site: http://www.maverickcomet.com/TechArticles/RemoveDash/RemoveDash.asp If you hover over the pictures, there are pop-ups with comments. Not the best web development form, I know...but I created this page around 2000 or so and have never had time to go back and rebuild it... Years ago I also reinstalled a dash without removing the windshield. It involved a lot of cussing and swearing and throwing things. I suddenly remember that in a fit of frustration during this project I stabbed a screwdriver into a cardboard box that was within reach and I had forgotten the instrument cluster was underneath it. Did some heavy damage to the clear plastic on the front and I think the speedo. This was back in the early 90s and back then there were still Mavericks in junkyards up here in Michigan and I was able to find a good cluster for something like $15. Still not good to be such a hothead...but I digress... From what I remember I was able to get the center screw in, but at least one of the far side screws I had to redrill the hole in a slightly different spot, which didn't throw the alignment of the dashboard off enough to be noticeable. I used long screwdrivers to try and keep the gasket out of the way while I used a small open end wrench to slowly turn the screw back into place. It was not fun, but I was able to do it. Just do some heavy meditation (or medication ) before you start...
Since you are wanting to replace the w/s seal anyway, why not cut the inside portion of the old gasket around the screws, then all is out of the way to get the screws out and then back in.
I also pulled a dash without removing windshield...but in the middle of the sweat-festival, I did see the wisdom of having no windshield prior to doing this. I am planning on powdercoating this almost pristine junkyard find (bargain at 14.00) and installing it in my car...was thinking about trying to use rivets instead of screws to remount, rather than having the windshield pulled. Anyone think that might be a viable option?
From underneath ...obviously it will be challenging... After removing the first dash with window still in, I was able to get to the underside of all mounting screws I could not remove from topside of dash and break them off with vise grips....so should be able to get similar access with rivet gun.
I'd venture to say that rivets would not be possible unless maybe the entire underdash is clear. I went in with hex head sheet metal screws. That way I could use a long extension and socket from the bottom to get them in and out.