Seems like many should consider using a fixed line for security while climbing steps. A fixed line and an ascender, or at least a prussik loop would be easy to justify. The ascenders are much more secure than a prussik, especially on wet or icy rope. They have little teeth that bite the surface of the line. Prussiks tend to slide a little before they seize up, and slide a lot on a slick rope, but they are better than nothing, and you can make them yourself.
I was thinking here of not using the steps at all, but instead, climbing the rope itself, plus grabbing the occasional limb as you get to them.
I could buy the rope in bulk. Cut and attach the line as needed to each tree I plan to use, probably a 30 ft piece would allow for knots at top and bottom. You would need to secure the bottom just so the rope doesn't just lift up when you are trying to slide the ascender up. Otherwise it is a pain, you often have to reach down and hold the rope below the ascender as you slide the ascender up. I suppose you could just tie it to your pack to weight the rope and then use the rope to haul up your pack, but then the pack is in the way/underfoot at the bottom. I'd hate to step on my arrows or bow.
I already have the mountaineering stuff from doing climbing on mountains/glaciers, but don't own any steps, so this is what got me to thinking this way.
With two ascenders, you can slide one while standing on the other, and go up a rope step by step. Loops of web from the ascenders (runners) give your foot a stirrup to stand in, so you can unweight the other ascender and move it up. This way you use your legs to climb, and your arms just to reposition the ascenders and guide you around things in the way.You are always clipped to both ascenders which are always on the rope, and you can add a prussik loop to the rope if you want one more attachment as a back-up, in case you do need to take an ascender off the rope to put it on again above an obstruction.
A dilemma is how to set up the fixed rope in the first place. I guess I could heave a light line with a weighted end across a limb and pull a heavier rope across with that, and tie both ends off to something solid near the ground. Then climb one side of that doubled rope to get up in the tree and set up the fixed line. Would have to improvise on site, of course. (Maybe my fishing arrow could carry the first line over the limb?)
I could come down by reversing the process, standing on one ascender and releasing the grip on the other enough to slide it down without ever taking it off the rope. Just alternate back and forth until I can stand on the ground.
This rope climbing stuff is old hat to spelunkers. There are karst caves near here where they have to do free rope descents/ascents at the entrance of over 100ft. They rappel down on the way in, and climb out with ascenders. Imagine the first guy doing that, into the dark! Put a big knot on the end of the rope so you don't slide off before you want to!!
If you are interested in this stuff, take a look at Freedom of the Hills or some other mountaineering book, or go to a mountaineering/rock climbing shop. Arborists would also be a good source for info, maybe even more directed to trees.