Looks like they have their own plugin framework. I've been watching the Dillo project for sometime, in 2005 I had feared it was dead, but it has been remarkably unkillable thank goodness!

Their change log is quite comprehensive:

http://hg.dillo.org/dillo/file/tip/ChangeLog

Refreshingly for me, they have tried to do a lot of documentation, which can be found here:

http://hg.dillo.org/dillo/file/devdoc