Star san ice cube
 

Is it possible to put 6ml = to 1/5th of a fluid ounce in a gallon water jug, freeze it and then cut it out of the plastic to drop into hot wort?

Sure it's possible...but why?

Boil water for a few minutes then freeze that.

Why do you want to put star san in your wort?

I just figured it would be a quick and easy way to chill down the wort without risking contamination.

Just a giant sanitized ice cube. Just the same as water right?

If you freeze it, the only thing Id worry about is chlorine. If you can use Brita or the like to clean that out, I wouldn't use any sanitizer. It would be a waste of money and risking flavor contribution or unintended consequences.

So if i use the sanitizer in the carboy and there is some left over, it may effect the flavor o the beer?

Well sanitizer/water solution (1oz to 5gal).

Leftover flavor from undrained sanitzer is far less concentrated sanitizer than an icecube formed from mixed solution. Semi scientific taste tests have proven that carboys that havent dried should not contribute flavor, but when you have a gallon or more of sanitizer solution mixing into the beer, I would be leary.