According to the blog, custom fields will be available soon, but I couldn't find it listed here, so I just thought it wouldn't hurt to add it. This is a critical need for my application. I'm using Sendy, by the way.
You can now add up to 10 custom fields per campaign :)
How does this work though. I've tried literally every possible combination of things that I can think of and I cannot get any data to pass over to Sendy - can you explain how to get some custom text passed to Sendy as the value of a hidden field please?