I have an exercise in a Spanish book (page attached) where two groups of words are separated by a spaced colon, indicating that the words on either side are to form separate sentences in the answer. Although this is not a dictionary, would it be appropriate to use the symbolic colon .3 here? I couldn't find anything about spaced colons in the Interim Manual.
I agree with your solution--use the symbolic colon given in Formats. Somewhere in the Interim Manual is a statement that says to follow the current Formats for formatting not covered in the manual. I would do that here. So list this in Special Symbols with the name given in Formats. I would also add a very brief explanation of use, such as the one you gave in your original post, perhaps something like Symbolic colon. Indicates separation of words into two sentences. Feel free to use your choice of wording.
And when the reader sees how this is used, he/she will get the idea. No need to comment about the spacing--just follow print for spacing. With the symbol identified and explained, the reader will discern the spacing from actually doing the reading.