May suggest a different mechanism for distribution, instead of captchas?

The idea is to make the distribution as balanced as possible relative to the number of people who are going to want to get free coins, and to their physical ability to solve captchas. This would also make irrelevant the automatic captcha solvers.

Make a small desktop / web application which does some proof of work based on a lot of RAM (like 2...4 GB). The idea is to make CPUs as efficient or more efficient than videocards and ASICs.

The result of a day of work (on an average computer) of this application would be a string which is sent to you through a website. This string contains the necessary information to validate the proof of work (in the same application).

For the valid proofs of work, you can then manually (or automatically) distribute the coins to the participants.

You allow this for a limited time, like a month, or whatever.

What you need for the proof of work:
* A (random) seed which you generate only immediately before you make the final build and release the application to the public. This will prevent precomputation.
* An address for the account which will receive the coins.
* A timestamp. This is used to generate different proofs of work for the same address.
* Possibly a counter, depending on the proof of work algorithm.

As you can see this is a like mining, but very limited in time (and therefore energy) and without the immense complexity of implementing one into the coin itself.