Mow or weed eat, maybe wait for a hard freeze, snow will flatten the grass.(shouldn't be a problem this year). If that's not possible use a small coil, try to find one with out spokes or holes that hang up on the grass. I use my 5.75 on a vaquero in heavy brush.
I've read before about guys bringing in old carpets, they lay them down over the grass and its like a mowed lawn. This would be another tool to think about, especially in areas that you cant bring a mower into.
Try steping on it while walking backwards keeping the coil close to your foot
you just want to kind of slide your foot a long. Not the best way to detect but
can be done, also try just pushing the coil down spot to spot instead of trying to
swing it as normal. Tall grass is a pain and there is no easy way to work in it.
Occasionly when I encounter it I just set on full disc. and run through it pushing it down with the coil. I find some larger items like watches and also coins on the ground surface. Plan is that if I can air test at ten or so inches I can get close enough to locate coins like this. Also if the wife would find out I was weedeating or mowing out there when I was neglecting the same at home...big owie.
Something I've used before is an old clear plastic carpet protector for an office chair or a square sheet of wood paneling. Drill two holes in one edge of it. Use 6 - 8 feet of rope through the holes and knot the ends as a handle.
Put the rope handle side up.
Push it to the ground with your foot.
Allows you to detect a 3' square at a time.
Lift it up, go 3' forward and repeat.
Slow going but can be worth doing if it is a good site.
Spewed my coffee laughing about 'purple shirt lady style'!! I use the same method mentioned above with the plywood and rope. I drilled some holes in the ply with a hole saw so I could stick flags in the ground (hopefully) near any targets. Mainly it's because I'm not patient enough to first tamp down all the grass and THEN detect!
On the mowing thing-you may gain points with landowners if you spiff the place up!