Code re-use would be great. The code will probably need some refactoring to accomodate that. And as far as i've heard Victor is willing to accept refactoring effort (patches) from external projects :).
Do you imagine that the user would have both OsmAnd and the other application installed? The Osmand instance would provide UI for data download/management and of course the rendering engine service. I think that's a goog way to go.
Tibor