Brewing coffee concentrate using both hot and cold methods - Freezing concentrate
I think this will give you some very disgusting coffee. By doing both a high temperature and a long extraction, you will get the worst of both worlds. 

High temperature extractions can get some great flavors out of coffee by forcing out a lot of the caramelized sugars, but usually they also use a very quick extraction to avoid also pulling out the bitter flavors.

Cold brewing works by remaining below the temperature that the bitter compounds are most easily extracted at and using a long extraction time to get out the most of everything else.

By using a high temperature and a long time, you'll get most of the sugar out, but you'll also very efficiently extract all the bitter compounds in your coffee. 

I would think your best bet for something like this would be to just use a standard cold brew method. Let the grounds sit in room temperature water from 12-24 hours, strain it through cloth (I like using a clean old t-shirt), and then refrigerate your concentrate. I also know people who have frozen the concentrate in ice cube trays to use in blended drinks, so it should freeze well too.