Hi Brian, re: radial projection method, what about the "fin construction" method as shown by Danny here:

http://moi3d.com/forum/index.php?webtag=MOI&msg=4084.3
http://moi3d.com/forum/index.php?webtag=MOI&msg=3530.2

That's where you sweep a line along your helix (with Twist : flat option set) to build the fin surface and then use Construct > Curve > Isect to generate an intersection curve between the fin and the rounded rectangle.

Although I guess now that I try that it's maybe not what you want, the intersection along the longer sides will be a little wavy since the object does not have radial symmetry.

So possibly an exact radial projection is not really what you want here?

- Michael