On making credits more valuable to contributors, If you styled them "extras" then the general public will be more likely to view them. This requires that the credits actually feel like bonus content. A music player would suffice for music. For tilesets, examples of the creators' intended use of them may work. Listening to and viewing all this content would take a long time, so --this is the important part-- combine the player and the viewer. The more sensory input you give the audience the less likely they will get bored.
I'm unsure of how making this available in game could work without breaking the mood/immersion. Having it accessible from the game menu and menu bar (in windowed mode) should be enough.
In a completely different direction, how about playable credits? The interactive game is the focal point of the project. Why not use that engine to display the credits? I envision a level designed around most of the created content with credits fading in and out telling who made what. "Music by ..." "Tiles by ..." "Monsters by ..." Everything could be included in this fashion including programmers and "special thanks."