I run a lower grill block most of the year on my '05 Prius, but I remove it in the summer because the inverter gets warmer than I would like on longer trips. You might be able to get away with it if you're doing low speed, short trips. I would definitely use instrumentation that shows the inverter temp. Upper grill doesn't get any blockage until temps are below 40F.
I would recommend using some AC to try to keep the battery cooled. A toasty battery probably won't last as long and won't give as good of performance.
An EV mode button can help in all weather when used as an ICE kill switch. With the way the
warm up stages work in the Gen2 Prius, you can easily get stuck with the ICE running when it doesn't need to be. The EV button can help fix that.