I want to create a cover page that has text over a background image. Is this possible in MigraDoc / PDFsharp?
Yes.
Background image can be a page from a PDF file or a "real" image (JPEG, BMP, ...).
Add the background image first at an absolute position. Following text will be rendered above the image.
See members Left, Top, RelativeVertical, RelativeHorizontal, etc. See also here (Images and TextFrames are both Shapes): http://forum.pdfsharp.net/viewtopic.php?p=3133#p3133