What I was thinking of was a widget-style app with two text fields, a button ("Go!") and a display area to show the route (in text form?), plus another button to bring up a high-resolution tube map if required.

The closures problem may be a bit more tricky. I was thinking of subscribing to the TfL closures email and parsing it, then putting a machine-readable version on a server somewhere. The app would then download this list of closures and implement it in its routing scheme.

Right now the app is not optimised at all, but if the demand is there I am willing to streamline the code as much as I can, which may not be much, and make the code for the "engine" publicly available ASAP.