Hi Todd,

The method I would suggest for you would be to make disperse dye iron-ons. Buy some Crayola fabric crayons, which look just like ordinary wax crayons, but are made of completely different materials. Use them to draw and color in a number of leaf shapes on paper (ordinary printer paper is fine). You could just do random shapes, or you could trace leaf shapes, either from real leaves or pictures you find online and print out. You can then use a hot iron to press the crayoned images onto your polyester shirt. Repeat this several times, overlapping the leaf shapes, until you like the effect. Use different colors, including brown and green, but possibly also black or occasionally a contrasting color, which in combination will give an even better camouflage effect. I like the idea of using leaf shapes because the overall result will be nice to look at closely.

Do not use ordinary wax crayons that are intended for children to draw with. You have to get special fabric crayons. They are just as easy to draw with as regular crayons. You can find these crayons at a fabric store, or maybe at a local crafts store; if you can't find them locally, you can order them online.

For more information, take a look at my page, "Iron-on Fabric Crayons for Synthetic Fibers". 

You can also buy disperse dye powder, mix it with water to make a paint, and paint the shapes onto paper that you will use for ironing on to your polyester shirt. Let the disperse dye paint dry into the paper before ironing it on to your shirt. This is more trouble to arrange, since you should order the transfer dye from a dye company such as PRO Chemical & Dye, but it is less trouble to do, since you can paint large pieces of paper in much less time than it takes to color them in with crayon. In both cases the method of dye application is by using a hot iron, so there is no need to boil the polyester shirt with the dye, as you would have to do to get a smooth solid color. The transfer dye powders are available in more colors than the disperse dye fabric crayons are.