In my professional opinion, exemption is not the same as exclusion. It may be that other load cases and combinations govern the design instead of "SDC-C with overall weights less than 20lbs".
PLEASE NOTE: I am use to dealing with structures supporting mechanical devices and including some frequency avoidance as part of the design. Mechanical, nonstructural devices are unfamiliar territory ... "grain of salt"
If unfamiliar, I would highly recommend a complete review of any laws regarding engineering practice as well as any documentation that may govern design which you may need to include and reference.
Customers/clients, in my opinion, may request additional work. You never know what experiences may be behind the request. I would include (and highlight) the request in my documentation as well as provide my client with written recommendation of the design requirements and note (if known) that it may be an additional step based on a review of the design requirements. However, any engineering efforts beyond the scope of work (SOW) should be identified to your client. These efforts may warrant revisions to the SOW, Estimation of Costs, and/or Estimated Completion Date (ECD).
Most documents serve as guides until you get into the "shall" and then you may be dealing with "minimum" design requirements. Remember, you can make design adjustments above the minimum that make sense to your specific case. Documentation justifying your approach is key. A peer review of your deviation prior to the design may also be warranted.
Humbly submitted.