I have written a class library using Visual Studio 2010 C# to read hardware information of a Computer (e.g. HDD/SSD). I will use this dll to create an installer using InnoSetup
Though its more than a year, I recently had the same problem and was able to rectify using the below script.
[Run]
Filename: "{dotnet20}\RegAsm.exe"; Parameters: /codebase YourDLL.dll; WorkingDir: {app}; StatusMsg: "Registering Controls..."; Flags: runminimized
If the file has be registered at initialize step, we can use one of the Inno setup's support functions.
function Exec(const Filename, Params, WorkingDir: String; const ShowCmd: Integer; const Wait: TExecWait; var ResultCode: Integer): Boolean;
More Info can be found in: Inno Setup Help