Thanks for those. They are pretty much what I was looking for. Don't know why but that info was really hard for me to locate via google when I last looked.
If you have the inclination you can see where I got to with this code because I have uploaded a new version here:
http://djcj.org/code/kluppe-0.6.14-playbackdelay-v2.tar.bz2
The core mod is in src/common/looperdata.c:1355
Basic operation is to create a new track, import a buffer file, load the buffer, set the playback delay to > 0 and press play. When it gets to the end of the loop range it will stop for the number of seconds in the playback delay spinbox.
It's now at least partially working. Needs some finetuning with multiple tracks but at least that annoying buzz has gone and the ui stays responsive. I'll spend some more time on it in the next few days no doubt. But if anyone else feels like giving it a tweak then be my guest. I'm sure Louigi will be keen to test out any improvements.