Since 91.417(a)(1)(iii) requires a certificate number in conjunction with the signature, we have a problem for the sign off unless we "cherry pick" those words out of the regulatory text, say it doesn't apply and not use it. However, IAW your own testimony above, homebuilts are not explicity exempted from 91.417, which includes para (a)(1)(iii). If the language said "signature and certificate number where applicable" you'd be home free. But it doesn't say that. Heaven forbid the maintainer is illiterate, I guess that's a whole new can of worms.
Maintenance recording regs become somewhat fuzzy when one tries to apply them to homebuilts. There is nothing that specifically requires it. We can certainly cherry pick regs out of context and apply them to support an argument, but that doesn't make it regulatory. Clubs are other organizations can say such and such is required but that doesn't make it so. There is no reasonable amount of civil proceedings to form a legal precedent, so it's 'swing and a miss' strike three.
I believe owners of homebuilts should voluntarily make a reasonable effort to track maintenance work on the plane but lets face it - what would happen if it doesn't happen? Nothing! How does the FAA take certificate action against someone that has no certificate?