I think this is related to what you are looking for...
I believe this is done by continually overwriting the prompt line with the ascii characters - \ | / (hyphen, backslash, bar, front slash, then repeat).
I don't think there is a way to do this through the api.
Every so often, your program will need to send a backspace character and the next ascii character (given above) to the command prompt. This will make it look like a spinning line.
The backspace character will erase the previous character, then the next character is shown. Then a slight pause, then another backspace and the next character.
Let us know how it works out.
(edited for spelling)
If you do a (princ "\r"), it will overwrite the current line of text, rather than making a new line.
Using that and manually incrementing the line you are rotating, you can get a spinner at the command line.
You migth need to throw another (princ) in to get the command line to update. In Lisp, it looks something like this:
(princ (strcat "\rMessage" spinner))