That could work. But unfortunately this form of OTP on a calculator is a memory hog.
The memory usage works out to basically ~57 bytes per character while it's processing everything.
Using compression on the lists increases processing time and I don't want to make it a lot slower than it is right now.

Short password lists (or math formulas?) could work, as long as you can protect the program, encrypted data and key themselves.
If only there was some faster way to use the sub( command, eh?