"Hmm, so even if the single tree had an emergency release the holdback straps would still be attached and pulling?
I'm thinking of a setup similar to this but for one horse. I have plans to only walk."
Hondo, while a lot of people use forecarts for recreational driving, they are basically intended as a means to attach modern agricultural implements with a team or single. The great drawback of using a forecart is that the balance of the vehicle is stablized by the weight of the implement. Without an implement as a counterweight, the weight across the horse's back can be tremendous.
If you are putting your toe in the driving water to test it, you might want to do some considerable research. I know how tenaciously you seek out information and understanding a bit about the mechanics of driving equipment will put you in a much safer place when selecting those first pieces of equipment. You can go to the Carriage Association of America, www.caaonline.com
and visit their bookstore (I even have a book there called "Understanding Harness"), or The American Driving Society (don't know their addy). Even your question about an emergency release shows you're thinking ahead about safety. The ADS site may list driving clubs in your area that offer introduction-to-driving clinics.
A lot of how-to books put the horse (training) before the cart (understanding the equipment) and I feel that may perhaps lead to less than optimal decisions when selecting vehicle and harness, which need to compliment each other. Needless to say, one of the best ways to avoid trouble is having a horse that is comfortable in its work environment.