Backup Certificate SQL Server and return bytes with a Select Statement
New in SQL Server 2012 and also in 2014 are two new encryption functions that will return a binary stream as you are describing. CREATE CERTIFICATE was also updated to take BINARY input for public and private keys.

CERTENCODED will return the public key portion of the certificate in BINARY form.
CERTPRIVATEKEY will return the private key portion of the certificate in BINARY form. this is encrypted so the password to decrypt it would also be needed.

The above could be saved in a binary form and written to a disk, or stored in textual representation of the binary form and written to disk.

Should you need to create it again, use the BINARY options of the CREATE CERTIFICATE function.

Edit: There is nothing in 2008R2 that can do the same that I know of through system functions.