Notwithstanding many other requests for selective snapping we've seen in this forum, some impassioned, some cerebral, there does not seem to be any way to constrain the behavior of the inference engine (by disabling certain snaps); the inference engine constrains everything else. I think you would agree that if you attempt to fight with the inferencing engine, it will win.
As far as drawing a line to the point of tangency of an arc or circle, that might be a somewhat dubious capability, since I find that as a concession to the segmented nature of arcs and circles in SU I generally fudge the tangent as joining at the closest endpoint. If there were such a thing as perfect curves in SU, I would certainly look for much more refined tangency inferencing, but given the overall blocky nature of curves, blocky tangencies aren't all that jarring, and the same mechanism that makes segmented and faceted curves look acceptable--smoothing/softening--also improves the appearance of the tangency. Just my opinion.
You might have much better luck tracing the outlines of 2D images used in "face me" components outside of SU. There are many (some) image-to-vector tracing programs out there that do a fairly creditable job, either automatically or semi-automatically. Then you can just import the vector tracing into SU and mount the image on the resulting face. I've had a certain amount of success using the tracing application that comes with CorelDraw.
There is also another option. Smustard offers a Ruby plugin called Image Profile that traces images for this very purpose.