Thanks for the question on the forum. Please let us know the version of AMT, BIOS, OEM vendor information and the tools that you are using. All of this information would help us better in answering your question.
I just checked and lower case works. I added a "sent" command with the string "AbC" with Intel AMT Outpost was running and it works. I also saved the script and re-loaded it and still ok.
By the way, I changed the file format for saving the script from a binary format to a text format in the next version. I still support the binary format, but with the new version you can edit the script with Notepad, etc.
Can you elaborate of give me any hints as to the problem... I can't see it.
Ylian (Intel AMT Blog)