Specifically, am I breaking the EULA by doing this?
Why bother using .NET Reflector? Just look at the original source code!
After more research, I found that the .NET Framework falls under the same EULA as the operating system on which it is installed, with a couple of additional terms which you can find by running the following command and choosing the right one for your language (1033 is in English).
where /R "%WINDIR%\Microsoft.NET" eula*.*
So as Microsoft operating system EULA's explicitly state that to "decompile or disassemble" is not permitted, and looking at code in Reflector could be considered to fall under this, it appears that technically it is not legal to look at the .NET Framework source code via Reflector. Even though you can look at the original code... an interesting paradox.