A W-9 is not proof of anything when it comes to employment. Even an unemployed person can fill out a W-9 and so long as the information is "true and correct to the best of their knowledge" they've done nothing wrong. If you are self-employed and your business is registered as an LLC or higher, the only thing that I would accept that would come anywhere close to "proof" would be a copy of your business license. I might also request a copy of your most current state registration as proof your business was in fact "active" with the state.
For example, you can get proof of the status of my own LLC at http://search.sunbiz.org/Inquiry/CorporationSearch/SearchResultDetail?inquirytype=EntityName&directi... and it clearly shows my business is "active" and the registration is current.
