The base idea behind JTD is to find a configuration of the JIRA Personal Edition allowing to perform GTD. The Personal Edition of JIRA is free for personal, non-comercial environments, so everybody can use it for the personal things to do.
The next idea is to extend JIRA in a way that the single personal editions of different people can communicate with each other. All communication will be by email, so the JIRA instances can be installed in a private environment not exposed to the internet. At first the following should be possible: When person A delegates an issue to person B an issue in person B's jira is created and when person B is finished with that issue person A's jira gets informed and the issue in person A's jira can be finsished.
Another idea is to have a plug-in that does all the boring set-up stuff inside JIRA for JTD so that installing JTD is as easy as installing JIRA and dropping in a single plug-in.