There are some epoxies that are UV resistant. The issue with using epoxies and resins, is that they have different expansion rates than the Corian itself. That can cause them to separate.
Most address signs, or any other signs, don't use fills. They use exterior paints, and for us, we've had success using automotive paints. They are designed for exterior use with UV inhibitors and resistant to most chemicals, especially any that would come in contact with the sign. There are several spray bombs that work well with Corian as well. I've used that method when I first started and they still look good, even after being outside, year round for several years.
Depending on the appearance that is desired, the paint can be applied after cutting, or as we do mostly, before cutting. There is a lot less clean up and after cutting, just clear coat and it's finished.
An inlay will give a flat surface, and in most cases, a dimensional surface looks better for signage.
Corian outside durability
Re: Corian outside durability
I have made a number of address plaques and one that sits beside a country road mounted to a post. It has been in the direct elements for years now and still look great. It is composed of inlaid letters as well as v-carved images that were painted with exterior enamel. I tested a number of epoxies and found the strongest and most reliable were those formulated for solid surface. These adhesives seem to fuse the pieces together as opposed to a surface bond. Hope this helps.