Daanji got you covered there.

Once you have the talent in Frost, it is Runic Power free.
And the ability is off of the GCD, so you can use it freely whenever you want even if another ability is firing off.
Either 4.1 or 4.2, mentions in the patch notes that interrupt effects will automatically hit regardless of hit rating, so we either have that now or will soon.

You want to make a mapped key for Mind Freeze.
Give the mapped key four steps, one per DK.
Each step sends your Mind Freeze keybind to one DK.
Check: do not advance to next step for at least 2.6 seconds.

Then imbed this new mapped key into all of your DK DPS spam keys.
Do Mapped Key > Mind Freeze > Target: Group (all of) DK.