I tried feeding a line mesh outlining a square into a Make 3D Object node but it didn't fill the outline. Also the corners were nasty, could do with some chamfering at least (a simple triangle calculation using line width).
I'm going to go ahead and make a FR seeing as Questions are rarely answered on this forum.
To a limited extent, yes — If the polygon can be created (or flattened from) one of Vuo's 3D shape nodes. You can see the nodes I mean by searching the node library for "make 3d shape".
For the example you gave of a filled square, you could use
Make 3D Square together with
Make Unlit Color Shader. Or for a gradient, use
Make Unlit Image Shader and
Make Linear/Radial Gradient Image.
These would just be simple shapes without other items you mentioned in your feature request, such as a stroke outline. (For others' reference, the feature request is here: Make a Polygon Mesh with Fill node.)