Some good thinking.
And some right thinking.
Steam uses the time on your steam page in order to coordinate it with your rank.
According to FourZeroTwo, VAC isn't taking extreme measures to combat rank hacks, but they will compare STEAM TIME to certain levels and prestieges to ensure its legit.
Basically, there are magical numbers they set that your time has to match up to in order to have that rank.
You will be flagged as suspicious and have a possible VAC delayed ban if you abuse this.
What I did:
Went to Level 70, Prestiege 1.
Prestieged, and played till 10-20
Rank Hack to 30.
Play to 40.
Rank Hack to 45.
Play to 50.
Rank Hack to 55.
Play to 60
Rank Hack to 65.
With this, it begins to match up.
Another option (a better one) is to simply leave your game on overnight!
Then, just add levels according to the time your game was on.
Again - I have no idea where the precision of the rank hack control is, but I know its there.