Have you contacted Tam Valley and asked them? You are planning to use the Frog Juicer in a layout in a way it wasn't intended. I don't want to guess and be wrong.
I thought you said you weren't going to use the crossover when using DC. If Tam Valley says you won't harm their juicer, one thing is for sure, the juicer won't be able to reverse polarity when DC is applied. A juicer can reverse polarity for DCC because the DCC power signal is a form of AC.
Wiring for DCC