In Inno Setup is it possible to add conditional statements to what in the [Run] section gets executed?

Alternatively is it possible to manually update the built in progress bar? Basically I have 2 MSIs included and using Inno Setup as a bootstrapper, and depending on user input one or both of the MSIs are to be installed. I have something working using Exec statements in CurStepChanged but it doesn't update the progress bar as the files are extracted and it looks like the installer is stalled. I guess the end result is I want some progress bar updates while the files are extracted to the temp folder. The following is my current code:

procedure CurStepChanged(CurStep: TSetupStep);
var
    ResultCode: Integer;
begin
    if(CurStep = ssInstall) then begin
        if(InstallServer) then begin
            ExtractTemporaryFile('ServerSetup.msi');
            Exec('msiexec',ExpandConstant('/i "{tmp}\ServerSetup.msi" /qb INSTALLDIR="{code:GetInstallPath}\Server\" ALLUSERS=2'),'', SW_SHOW, ewWaitUntilTerminated, ResultCode);
        end;
        if(InstallClient) then begin
            ExtractTemporaryFile('ClientSetup.msi');
            Exec('msiexec',ExpandConstant('/i "{tmp}\ClientSetup.msi" /qb INSTALLDIR="{code:GetInstallPath}\Client\" ALLUSERS=2'),'', SW_SHOW, ewWaitUntilTerminated, ResultCode);
        end;
    end;
end;