Here is just a small thought:
I am not an app programmer, but I can imagine that actually developing a very good feeling hand sketching functionality comparable with established dedicated sketching apps on the market can be a big task (witness how few of all the sketching app offerings “get the feel right”).
Rather than focussing on such a new development from scratch it may be more feasible to develop a Rhino for Mac plugin to directly support established sketching app data first and coming up with a well rounded expanded Rhino iOS later.
I am looking especially at Concepts app here, as to me from all the more advanced sketching apps it allows for the highest performance currently (both in feel and accuracy of the sketching and in its features towards more technical applications rather then the generally “artsy” approach of most offerings).
Concepts also already allows for the most extensive I/O in file formats and has all iOS <-> OS X file transmission options already established and working stable.
A Concepts - Rhino plugin would be something I would love to pay money for if it supports layers, accurate curves and some flexibility re. plugin settings (to assure best quality data to work with).
This could be a stop gap solution to bring iPad sketching capability to Rhino for Mac and it could be very, very useful even if an expanded Rhino for iOS is in the future.