By the way, Kim, could you tell me what the feedback loop is? (what kinds of operations are you using?.... is it linear?...etc...) If it's linear, you should be able to replace the feedback loop with an equivalent operation, which circumvents the whole problem.
The structure I am implementing is basically a modified model of the structure shown on this page - http://ccrma.stanford.edu/~jos/swgt/Rigidly_Terminated_Ideal_String.html
I have this model working (by using a delay with length 0 and blocksize set to 1), if you're interested it's here http://www-users.york.ac.uk/~kt503/PD/kt1-d_DWG-a.zip (unzip to folder, open 1-Ddwg-2g.pd)
However I now need to integrate this with other modules on a higher level (this is just a simple component). The idea is that the delay line is bi-directional, and at the terminations they always form loops, so as far as I can see it can't be implemented without it... K