Recently I've been playing around with some variables of training, using a similar outline of HCT-12 and Myo-reps and the combination I'm using now is working wonders. I figured I'd post it here for anyone looking for a twist on this plan.
I divide the workouts the same as HCT-12 as far as movements go, and alternate between a "heavy" day and a "light" day. Although both are hard work, the differentiation is simply that I'm using lighter weights and higher reps on one day. Heavy day is 6-9 reps, Light day is 12-15 reps. So a 4 day cycle would be:
Heavy Lower (6-9) - Heavy Upper (6-9) - Light Lower (12-15) - Light Upper (12-15)
This alteration in rep ranges is different enough of a stimulus to allow you to work hard, while not overdoing one capacity and running into the ground. On a 4 day/week system, this means after a heavy day, I have a week before I hit that group heavy again, but I take advantage of the time course of protein synthesis by hitting the muscles again a few days later, but differently enough that I'm not overworking myself.
Ramp up to the top set (less ramping is needed on the light day) and then depending on your rep range, do clusters of differing amounts (this is the myo-reps aspect). Heavy day, do clusters of 2 until you start to get grindy, drop the weight and do clusters of 3 until you get grindy. Rest a minute or so, drop the weight and do one last back off set for max reps. On light day, follow the same pattern, but do clusters of 4 until you get grindy, then keep that weight and do clusters of 3 until you get grindy. Again drop the weight, rest, and do one big back off set to max reps. Do this for each movement and you're done.
The beauty of the clusters is that they give you more growth stimulus with less fatigue, which is critically important to keep growth going and recovering. Because the altering of the rep ranges, I've actually run this system on a 6 days/week plan and noticed great improvements. If you have a busy life, I wouldn't recommend doing 6 days a week because it gets to be too much, but if you have a slower schedule or want to splice in a few weeks of high frequency, it can really be great. I'm noticing good gains on 3 or 4 days per week so it's really your call. Just know that higher frequency means you need to stay farther away from failure on your sets.
This may sound complicated because of the way I explained it, but it's pretty simple when you do it. It's a nice twist on HCT-12 because you get to hit different rep ranges and different clusters.