How may I cut the charging rate in half

It is not the CABLE that knows that.

Explain again why you want to limit the charging rate ?

If you connect it to a source that is not capable of supplying the 11 amps, I would hope that the charging controller would be "happy " with less.
Others with some actual experience can probably provide more information.

 

Few things here...

First off the charger is actually built into the car, that's what's requesting the amps. The cord you're using is an EVSE which is basically a smart extension cord. It basically just connects been the car and power supply and says "is everything ok? Yes? Ok I can supply this much power, here it is.".
But the default EVSE that comes with the Prius and the charger in the Prius have no amp limit options that I'm aware of. Some cars like Volt have options in the car charger to use less amps.
Or some EVSE's like mine from openevse have options to only offer the car less amps as well.
The only other factor you might want to keep in mind is that there is charging overhead just to keep the car computers, etc powered. So when you get down to like only supplying 2 amps you might be losing a quarter of that too overhead and so you end up with a lot of hours of extra energy used.

 

Your Owner's Manual gives instructions for limiting the charging current to 8 amps. Here is a snippet from the prior year's OM:



I have never heard of any other choice for reducing charge current.

 

welcome!
i have never heard of anything like this being done before. you may be the missionary, all the best!