Is it possible to enclose a font file in a Seaside application so that the client renders the website using that one instead of the ones in their system?
If so, how would one do it?
This is more a question about CSS and using @font-face; see this article, for instance.
About the Seaside side of things, the font is just a static file, so the usual ways apply; for instance, you could include it in a file library and possibly as a static file served by the front-end http server for performance.