Went ahead and ordered a 4-relay module and a set of 12V green lighted switches as you recommended above. The HiLetGo ones weren't available in the 12V 4 channel module, but it appears that may of these modules are the same thing sold by different distributors. The one I got was a "Vogurtime" brand.
@Mike H Mottler: Good suggestion, and I'll be using some of the classic Lionel lighted bumpers at the ends of my yard for an additional indication, but that won't work for the power zones that are part of my loops. I'll probably add some basic 14V lamps near the power zones on the layout to indicate that the relay actually flipped and is feeding power.
Great suggestions, guys. Will let you know how it pans out!