Hi...
I don't think that "forgetting" your certificate is covered in the Regs, however, the excerpt below is specific on obtaining a replacement if necessary. Nothing is mentioned about using a copy.
§ 61.29 (e) A person who has lost an airman certificate, medical certificate, or knowledge test report may obtain a facsimile from the FAA Aeromedical Certification Branch or the Airman Certification Branch, as appropriate, confirming that it was issued and the:
(1) Facsimile may be carried as an airman certificate, medical certificate, or knowledge test report, as appropriate, for up to 60 days pending the person's receipt of a duplicate under paragraph (a), (b), or (c) of this section, unless the person has been notified that the certificate has been suspended or revoked.
Regards