Definitely not. You're unsafe, in my opinion, if you risk it.
Do a test -- get up to 30mph, and simulate a panic stop: lock up your coaster brake and see how long it takes to stop. I assure you the distance is unacceptable.
Add a front brake, and you'll increase your safety immensely.
I have a caliper style brake. Don 't like the looks or the (lack of) effectiveness. I'll be taking my bike down to the shop or a drum brake install on Saturday...if it doesn't snow again. :-/
I think a drum brake might suit the barebones look you're after. Give it a look.
Brenton