As Eowyn said, acting lacking corporals exist in the reserve. Knew one who didn't even have his QL3 mod 2 (sig op). Promoted two years after enrolment date, completed his mod 2 twelve months after that, then deployed overseas as a corporal.
Well, deployment would be different; Class C is treated as RegF in most respects, acting/lacking rank included. A Cpl I worked with was deployed as an 'acting' MCpl, but reverted back to Cpl afterwards because, as she was told, "There is no acting/lacking in the reserves". I also work with a Pte who was a Cpl with 4 yrs and her 5's in the RegF, who released and re-enrolled in a different trade but was offered Pte because she did not have her 5's in the new trade. When she questioned it, she was told, "There is no acting/lacking in the reserves."
Interesting that the Army and Air Force differ on this, though. I wonder what the Navy's policy is on acting/lacking ranks.
Does anyone have a ref (pub) handy on the differing policies for acting/lacking in the reserves?