I love this idea!
This website
$1 and probably others will transcribe a text letter and print it into Braille for you. Maybe you could write her a note, but change the letters so she will have to decipher it with a key. ie “A = F” etc... You should be able to type in the encrypted message and key with standard text and have it printed up in Braille.
I could see that being a fun challenge, but will definitely require some effort.