You are correct, there is not a way to do this currently within a single style sheet. You could do a save as of salembook.style and give it a different name and then configure it to not have numbering for bookmarks. Then you would use that style sheet when composing. It would be an enhancement to have this ability within a single style sheet.
Enhancement Requests should be submitted through PTCs Enhancement logger. Using this tool, the requests go directly to the Product Managers and there are several customer only dialogs that allow you to more thoroughly explain the Enhancement. The tool also lets you track the Enhancement in order to monitor its progress.
Your product ideas are important to us and we encourage you to submit them at the “Ideas” site located on PlanetPTC Community (communities.ptc.com). This site will allow you to submit your ideas, see other customer’s ideas, vote on the ideas that have the most value to you, and also provide additional feedback and usage scenarios. This provides PTC Product Management visibility into your ideas, as well as understanding the ideas that have the most interest across all of our customers, helping us make better decisions for future product capabilities and enhancements.