Define a multilevel list. For level 1 put the number as three spaces. Ensure that there is no indenting and that the symbol to follow the number is set to nothing. Link the level 1 to the style Normal.

Interesting and ingenious idea. I don't think I would want the Normal style given an outline level, myself. That is going to really mess with the document structure.

In addition, although it may be three spaces in the definition, there are not three editable spaces in the text, just the block of space.

I would recommend using Graham's macro instead. It is less likely to result in other issues both with your document and your publisher.

Here is the definition dialog which you reach by pressing the "More" button.
00 deleteme 1.png

The real problem here is that the publisher doesn't know how to use their publishing software properly so pushed the problem onto authors.

I agree, however, this is like the situation of the "boss" who wants things done his/her way. There is a terrific power imbalance.