I like board presses. My weak point is a few inches off my chest so the 3 board is what I use most. I'll use the board press to get in additional volume of a weight I would struggle to continue using. For example, let's assume I can bench press 1 top set (3 reps or 5 reps or whatever) of a certain percentage (80+% for example). A 2nd set may be doable but hard. A 3rd set is possible but no 100% guarantee. A 4th set, all the stars would have to be aligned for me to get it, and 5th set is a pipe dream. Instead, after my top set, I'll keep the weight on the bar and do a few additional sets with a 3 board.
I've never applied board presses to a Planstrong Plan. But here is my thinking on this matter... Plan your program like you normally would. Let's imagine this is a 2 month program. In Week 1, maybe your max Intensity is 85%. Maybe the 1st or 2nd set, do it per normal. For the remaining sets, use the appropriate board. As the weeks progress, use the board less and less for the 85% intensity sets. By the time the 2nd month starts, I would not be using a board on 85% intensity zone.
Another tool, the reverse band (green mini band, unless you are benching huge numbers)... Early on, I would use this for all my heavy lifts (92.5% RM for example). Likely, even with the board. I like using the reverse bands the 1st or 2nd time I introduce a new weight %. For example, maybe you havent lifted above 90% in a while (it helps me get accustomed to the weight). After 1 or 2 training sessions with 92.5%, the RB gets removed. 1st 92.5% set is normal, all subsequent sets are 3 board. Over time (the 2 months), the board gets used less and less (and likely not at all during the peaking month).
I like using RB's for squats and chains for deadlifts, too. Just be careful using these tools close to a max attempt. Or to be more precise, be careful using any assistance excersise that is similar to your main lift, 1 month prior to maxing. It can really mess with your groove and you may not even be aware of it. Use the tools, accessories, and tricks to build general strength and help with sticking points. As you peak (weeks or a month out), your training should be simpler and more basic, focusing on technical perfection (neurologic, greasing the groove) and realizing the strength you built in the previous months. You dont get stronger in a peaking cycle. Instead you are just realizing the strength you gained in the previous months. For example, I only peak when I compete at a meet. I dont compete very often (last one was September 2019). So why would I waste a month peaking (aka not building strength). I view peaking as a waste of time (but a necessary evil to prepare for a meet). <end rant, sorry>
These are my thoughts on this topic. Keep in mind I have limited experience with PlanStrong and no experience applying assistance lifts with Planstrong. I took the 2016 Planstrong class in Pittsburgh. I feel more comfortable with traditional Western periodization. As you try this out, please report back. I am very interested in learning more how this works out for you (because I'm sure it will apply to me as well). Dont forget though... There must always be a reason for doing something. Dont just add more reps for the sake of adding more reps. The board press volume needs to be included in your intensity/volume planning.
Regards,
Eric