There are 2 issues here I think.
- It takes a lot of time to prep a launch.
Replacing the intended cargo with something else can't be done on short notice, neither can it be done with people noticing something is wrong.
Launching it into anything other than the intended orbit would also be a big give-away.
- It might actually be illegal under international law for the spooks and the military to covertly use civilian launches.
Even if it isn't they'd get a lot of flak from the international community if they were found out.
And they would be found out.

Also, launching a satellite to get a closer look at DA14 doesn't make much sense, unless it's a real close look.
IOW a high speed flyby.
I'm sure NASA would love to do stuff like that, and proudly announce it.
If only people were willing to pay for it.
NASA can't spend a penny without prior approval of Congress.

As I said the spooks (NRO) and the military have their own launches, which they have to announce in advance.
On what it is they are launching they tend to be vague or perfectly silent.
But usually we can make a educated guess.