Is there a list of the built-in rbxasset textures?

No official list that I know of, but you don't really need one. rbxasset:// just points at the content folder inside your Roblox install. Open %localappdata%\Roblox\Versions\, go into the newest version folder, then content\textures, and that's all of them. Whatever path you see after content\ is what goes after rbxasset://. They get renamed or removed between updates though, so I wouldn't build anything important on one that Roblox doesn't obviously use itself.