Water pressure changes by 4psi for every 10 feet in elevation change. Given that the typical drip irrigation system starts with a 25psi pressure regulator at the hose bibb and the typical drip emitter only needs 10psi to operate properly, the loss of pressure because of the difference in height shouldn't be a problem.

If not designed right, your bigger pressure issue can be pressure losses due to "friction" of the water flowing through the pipes/tubing. These friction losses increase as the pipe/tube size decreases and as flow rate (i.e. the amount of water) increases.

To help hide the tubing, you're likely to be using 1/4" tubing to get water to the window boxes. Where you would likely run into problems would be running one 1/4" tube to feed all six water boxes. Your best bet to avoid water pressure issues would be to use 1/2" drip irrigation tubing to get near the boxes, and then run separate 1/4" tubes to each box, keeping the length of each 1/4" tube under 20'.

That's about all the help I can try to offer without having a picture to understand the exact situation to offer any suggestions on how to run the tubing.