I think that's the first time I have seen that request in over twenty years of using RoboHelp so good luck with finding an automated solution. 
Typically it is used in the legal area but more in topics than in the TOC. It is used less as time goes on.
Perhaps just manually number the top level books?
If you do find anything, please post back. Sorry it's not the answer you want.
See www.grainge.org for free RoboHelp and Authoring information.

@petergrainge
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