I am zone 4 - last frost Memorial Day, first frost as early as August 27. I have several volunteer tomato plants that have come up the past few years and they have definitely been my strongest, healthiest plants. We plow and till the garden the weekend we are planting. So they do not have a head start. As for peppers, the "jungle" idea is a good one. I started our seeds here very late. THey have only been in for 2 weeks. They are huge though (inside this is). We mist them, have boxed them in so they have no draft and are warm. Try seeds in the garden - what have you got to lose. Wall of waters - saran wrap over the seeds until they are up. Foil under the plants in our area to increase the heat and sun, increases our yield - it may cook your plants. Use your common sense and it will work.
We did renovations on our home one year and I could not get my garden in until July 4th - I was raging at the excavator to get that dirt off my garden for a month. The combination of the later start with really warm temps and good luck I guess - we had a great garden.
On another point - you can't buy any, you have the seeds, what have you got to lose. Good luck and don't sweat the small stuff.