Disclaimer: I am in now way a builder or contractor and take no responsibility for my actions or words.
At Home Depot/Lowes they sell a cylinder concrete form that you can cut to size. Could you put one in the 1.5' deep hole, leaving 1.5' of the form above ground and fill it with concrete, and also around the base for support? If it makes you feel any better, before the concrete hardens, you could tie something solid to some wire, then submerge that in the concrete on one end and tether it to the ground on the other end as sway support. Not sure if this would work or not. Guess a lot depends on how heavy the plants are and wind factor.
Here is link to the forms. http://www.homedepot.com/buy/sakrete-12-in-x-48-in-concrete-form-tube-65470062.html#.UQmTi7_C2y0
I was researching simple hop trellises and I like this idea. I was looking for a method I could add pulleys to it, so at harvest time I can bring the plant down instead of using a ladder. I'm curious about this as well because my ground is very rocking about 2' down.