So, I've been thinking about this, and ultimately I don't think I'm going to do it.
First, as far as I'm aware, you can't blueprint something that is not in your own force/faction. As the saplings are a part of the neutral force, this is why you can't blueprint them. Yes, there are shenanigans that could be done to circumvent this, but no, I have no desire to actually do that in part because I don't want to go through the trouble of it. Additionally, by making saplings a part of your own force, I believe this will cause them to become valid targets for biters, which of course just feels wrong to me.
Ultimately, though, allowing the saplings to be blueprinted I feel like would render the planters almost obsolete and thus what's the point in having them (and I spent soooo much time on their code...). Yes, without another mod (like recursive blueprints? Don't know; never used it) they wouldn't be 100% automated, but you'd be able to pack them in so much tighter than what you normally could without the need of the planters, belts, inserters, etc. Plant huge swaths, wait a couple days, deconstruct. Rinse-repeat, and you'd get many times more the amount of wood for the same space (or use several times less space for the same amount of wood).
If you're simply wanting trees to refill in areas that you're no longer using/have cleared the pollution from, there are two different modes for snap placement of the trees that you could use, allowing you to run up and down in the areas where you want to plant trees, allowing you to reforest that way if you want to avoid having to do a lot of clicking (after all, this should be a one time/rare deal). Alternatively, you can also use a mod like Noxy's Trees, Natural Tree Expansion, or Treeees that allows trees to naturally spread and grow and reforest that way, with maybe just sparse planting of some trees yourself to help it along.
If you're not, and this is simply so that you can use blueprints to plant a bunch of trees to grow, harvest, then re-plant, I'm sorry, but that was the whole point of the planter boxes. In my mind, and thus how I plan on maintaining this version of the mod, you have two means of growing and harvesting wood at your disposal: use planter boxes for 100% automation at the cost of several times the space a single tree actually takes up, or manually plant and use a deconstruction planner to harvest with the only space lost being the 4x4 of the roboports and the 2x2s of the big power poles.
(The space using a planter would consume would be roughly 8-11 tiles per planter: 4 for the planter itself, 2 for inserters to put in and take out, and then either 2 for the belt running past (where the saplings and harvested wood use separate lanes and the inserters are burner, making 8 tiles), or 4 for the separate sapling/wood belts running past plus space for a power pole to be inserted in (as at least the inputting inserters would need power, making 11 tiles).
Manual planting using one of the snap settings, for the space the planter setup uses, you could plant and harvest 8-11 times as many trees. No snap, as a tree is 0.4 tiles square, you could pack the same space with up to 50-68.75 times as many trees. I misremembered the size of trees, here. They're actually 0.4 from center, meaning 0.8 squared, but I also made the planted seed for the saplings slightly bigger at 0.9 squared. This means that using no snap, you can pack them in only slightly better than snapped, at just under 10 to just over 13.5 trees in the same space. No where near as bad as I was thinking originally, but honestly even the 8-11 times figure using snap is enough to me.)
