Ugly hack

[Types]
Name: "full"; Description: {code:InstallTypeDescription|full};
Name: "compact"; Description: {code:InstallTypeDescription|compact};
Name: "custom"; Description: {code:InstallTypeDescription|custom};  Flags: iscustom;
Name: "upgrade"; Description: "{cm:Desc_upgrade}";

[Code]
function InstallTypeDescription(Name: string): string;
begin
  if Name = 'full' then
    result := SetupMessage(msgFullInstallation)
  else if Name = 'compact' then
    result := SetupMessage(msgCompactInstallation)
  else if Name = 'custom' then
    result := SetupMessage(msgCustomInstallation);
end;