I would take Bonita as a model (not Activity diagrams but Business Modeling Notation):
http://www.bonitasoft.com/
It is open source.
I would not make it complicated:
* No nested swimlanes
* Just horizontal ones, no vertical
* Maybe you can adapt something what you already have
* Could be just two lines with a vertikal textbox on the side on the left
I use bonitas Business Process Modelling for source code! very often. I misuse it for this purpose. And it helps me a lot.
Everything becomes complicated in software development and my impression is in the last years more than before.
I cannot think that a programmer can cope with this situation without some modelling help.
And your tool is great for this purpose! (Very fast. I use gvim (on windows), F5 generate -> voilà).
Thank you very much. Ich will propagate it to others ...