Would you have a smaller tank, like a 10g for the platys to live in until your pregnant one is done dropping fry? Or maybe a tank divider to make a platy area? A seperate 10g would make a "fry search" easier - those little girls can hide really well and the less fish around to eat the fry, the better.
Either way, what we've done for our pregnant Twin Goldbar is taken the area around the heater and planted it as thick as we can with the tall fake plants. The rest of the tank is pretty open and I think this encourages Daisy to go into the 'forest' to drop her fry. The closeness of the plants also makes it hard for the other platys to get into the forest and go after the fry. We've covered the filter intake with netting from a fish net, using a rubber band to secure it in place so no fry get sucked into the filter. Right now we have the platy tank done with gravel because of an upcoming trip, but usually we have those flat marbles instead of gravel in their tank. They make for more tank cleaning (food & poop slide down to the bottom easy), but they also give fry a great place to immediately hide in the many gaps in the marbles.
When she's started dropping (actually when we think she's started), each day i'll carefully take out the tank decor and the marbles (putting them in a cup in the tank until it's full, then dumping them into a big bowl).. keeping an eye out for fry.. Usually, we'll remove the adult platy to a big bowl but if you had a divided platy area in your big tank, they could go on the other side of the divider even. It's amazing, even when EVERYTHING is out, fry still magically appear after i think i've found them all. It's a little more work, but we found 18 fry last time as opposed to 8 the first time.. and they're said to have less each time than the time before so we must've lost a LOT the 1st batch. We'll also take the opportunity to clean the tank, watching for fry as we do, before putting everything back.
After finding the day's fry, we'll
acclimate them to the 10g nursery tank then let them in to swim with the rest of the fry.
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I hope they help some with you coming up with a good plan for your setup there to get as many fry as you can.
