I am a US Citizen flying into Hong Kong and want to use the 72 hour visa. Is that possible?

Hong Kong don't have any TWOV program, as a US citizen you can enter HK without a visa.
If you wanna go to Guangdong province, you need a tourist visa if your route is HK-Guangdong-HK.

Or you take a flight to Guangzhou (just a flight is valid, no train!!!), use the 144h twov and leave Guangdong province to Macau (you can take a ferry but need the ticket in advance).