This is also unfair for miners...
Just my 2 spliffs
As long as it is a relatively small but not meaningless number of coins, like the 20 SPL being given away in the current twitter promo, the benefits gained from increased exposure would outweigh the negatives of some people dumping their coins ASAP and unfairness to miners. It is irrelevant though, because niktitan132 just did a pretty good job of illustrating why this idea is unworkable.
It was a thought anyway.
EDIT: He edited his post but basically it would take a lot of time and effort to verify everyone's trade records.