As far as I know, there is no {cm:...} like constant by which you could expand a [Messages] entry. If I'm right, then it depends on where do you want to use such constant. If it's in the scripting part, then you'll need to use a scripted constant with a getter calling the SetupMessage function, by which you can expand those built-in messages for selected language by the constants listed in this file.
As you can notice, every message constant has just the msg prefix of the [Messages] entry from the language file.
For instance, to expand the WizardPreparing message into the Description value for the [Run] section entry you would expand the msgWizardPreparing constant this way:
[Run]
Filename: "MyProg.exe"; Description: "{code:GetDescription}"
[Code]
function GetDescription(Value: string): string;
begin
Result := SetupMessage(msgWizardPreparing);
end;
In the [Code] section is the situation naturally easier, since the SetupMessage function you can use directly there. So, for instance, to show a message box with the expanded CannotContinue message you'd expand the msgCannotContinue constant simply this way:
[Code]
procedure InitializeWizard;
var
S: string;
begin
S := SetupMessage(msgCannotContinue);
MsgBox(S, mbInformation, MB_OK);
end;