I think I got it this time! I base-dyed this shirt, rinsed/laundered/spin dried, then rubber banded top and bottom in a loose accordion fold. Then I vertical-wrapped tightly with sinew, soda soaked, squeezed out the excess solution. Lastly, I painted the darker color on with an inexpensive house painting brush. After letting the shirt 'batch' for about an hour, I untied and did the rinse/launder dry bit. I think by painting on the dye, it limited the amount of dye, and the sinew (tightly wrapped) blocked the dye from soaking through too much, and created the 'bamboo' markings I was trying to get.

I also think it helped that this shirt is made of a thinner fabric than standard t-shirts, but I'm sure I will be trying with them as well, so we'll see!