<p>What should I do if I want to release a .net assembly but wish to keep its internals detailed in the manifest private (from a utility such as <a href="https://learn.microsoft.com/en-us/dotnet/framework/tools/ildasm-exe-il-disassembler" rel="nofollow noreferrer">ildasm.exe</a>) ?</p>

I think what you're talking about is "obfuscation".
There are lots of articles about it on the net:
http://en.wikipedia.org/wiki/Obfuscation
The "standard" tool for obfuscation on .NET is by Preemptive Solutions:
http://www.preemptive.com/obfuscator.html
They have a community edition that ships with Visual Studio which you can use.
You mentioned ILDasm, have you looked at the .NET Reflector?
http://aisto.com/roeder/dotnet/
It gives you an even better idea as to what people can see if you release a manifest!