I saw in this article which preached the benefits of concentric only training, and your mentioned in it:
Which leads me to this, the Max Pro Portable Cable Machine. It only gives concentric resistance, but I recall that Glen Pendlay had really good things to say about concentric only weightlifting.
I don’t know anything about it nor have I tried it. So I can’t really recommend it.
BUT I will tell you that my view of concentric-only training is as follow:
It is INFERIOR to reps combining an eccentric and concentric for strength and hypertrophy.
The main use for it is as a supplemental training means to increase the volume or frequency of training a muscle without increasing the recovery period much.
It can also be useful in a deload: replacing exercises by concentric-only training to speed up recovery. OR in-season for an athlete to maintain strength and size without negatively affecting practices and games.
But except if you are a professional athlete and have a lot of budget, I wouldn’t bother with that piece of equipment