Steve Daulton and Roger Dannenberg have both promised to talk about making your own plug-ins using Audacity's built in "Nyquist" scripting language at Audacity Unconference, 11-12 July, 2014, at UCLan (University of Central Lancashire) in Preston, England.
Steve is part of the Audacity support team and author of many plug-ins, Roger is an Audacity co-founder and the author of Nyquist.