Give the amount of material that ACE sent me to prepare for the pt exam and the amount of knowledge required, it seems irresponsible and also it would be putting yourself and your clients in a legal quagmire, if something went wrong. You would definitely be kicked out of the gym I go to! The trainers there all have degrees in exercise physiology, or a degree in something else and a certification. That's the route I plan to go.
I worked for many years as an aerobics instructor. When I first started, the certification process was new. I was in the first group of people to be ACE certified in group fitness in 1986 . After a couple of years, no club would hire an instructor without certification. The liability is too great. While I fully agree that the populations you mention are underserved and have health issues because of it, it just isn't right to have these people pay you before you are certified. Personal training is a medically related profession. In order for trainers to be taken seriously, they need to be qualified. Perhaps you could find a gym where you could work under some type of internship arrangement while you are studying?