This was done in the UK back in the 90's when the 'Euro' debate was going on.
A group stuck pink stickers to the 1 pound coins saying 'Save the Pound' which then spread through currency circulation.
This was brought up on a Facebook FT group about stamping US currency with something relating to the FairTax.
Someone is already marking over 'Tim Geithner' with 'TAX CHEAT' on the bills.
Now according to US law it is illegal to deface or destroy currency with the attempt to make it unuseable, but there is nothing stated with regards to marking currency.
What do you think about using the free market to advertise the FairTax?
Everyone likes money and everyone uses it......plus the cost would be just the cost of the stamp and thats really it.