First of all: sorry for the late answer! I had an extremely eventful day yesterday, and did not find one calm minute in front of my mac ...
The short answer: Unfortunately, Sparrow does not have that feature currently.
The long answer:
I'd love to have this feature myself; I use it regularly in Flash, as you can use it very elegantly for all kinds of stuff. However, there are several problems with this. As you will know, Sparrow uses OpenGL for rendering, and OpenGL is, in a nutshell, built to display triangles with textures mapped onto them. This is what is does best, and it is very fast with this task. Flash, instead, was built from the start up with vector graphics in mind. The advantage of this approach is the high flexibility -- the drawback is the relatively poor performance. Flash cannot use the graphics hardware very efficiently because of its vector-approach.
Thus, Sparrow exchanged this flexibility with the high performance gained by OpenGL rendering.
Everything's not lost, though.
You can still draw vector graphics into a bitmap, and then use this bitmap with an SPImage. As you said, UIKit provides a lot of vector drawing methods for this task. In fact, Sparrow uses this feature already: SPTextField uses UIKits drawing methods to draw text to a CGContext, and then creates a Texture from the context.
It's not especially elegant code, but you could definitely do that. Just have a look at the method "createRenderedContents" of "SPTextField" -- it shows you how to create a device context and then convert it to a texture. I might have a look at wrapping that code in something that's easier to use.
However, as cool as the "graphics" feature of Flash is: in nearly all cases, you will find a solution with pre-rendered textures. It might not always be as elegant, but it will definitely be faster (you can access them via texture atlas and spare the time creating the context and drawing the image).
I know, these are not the best news -- but I hope that helps you at least a little!