After playing around with WSPR, QRSS and RTTY on the Arduino, I decided to have a go at Hellschreiber based on the Arduino AD9851 project.
I can take no credit for this code. I simply glued Peter Mark's AD9851 code together with Mark VandeWettering's hellschreiber code and put power on it.
Source Code for AD9851 Hellschreiber Transmitter
It works wonderfully. It takes a standard string in, and sends hellschreiber via an Analog Devices AD9851. Exactly what it says on the tin!