There is a bug open requesting feature parity between the Cat6K and the Nexus when it comes to the CISCO-IF-EXTENSION-MIB (CSCsr21316). The bug is currently being worked on, but there is no ETA on when support will be added. One other customer reported this same problem, but no good resolution was found. I wonder if you might be able to get this data using the dot1dBasePortIfIndex from the BRIDGE-MIB. I do not have a Nexus on which to test, but it's worth a shot.
But unfortunately, according to the page on Nexus7K MIBs, the BRIDGE-MIB::dot1dBasePortTable is supported in 5.0(2), not in 4.x -- which does match my test. I guess I'll have to wait for a change to upgrade the code.
Your finding does compute with my NX7K configuration. The port 0x389 maps to port 7/9 with your formula.
So I guess, the formula is:
slot = floor(cieIfDot1dBaseMappingPort / 128);
port = cieIfDot1dBaseMappingPort % 128;
128 is likely the maximum number of ports per line card at Cisco.
The thing is, without Cisco documenting 0x0093 means port 1/19, we can only call that a guess. If the BRIDGE-MIB is availabile, then the dot1dBasePort number is mapped over to ifIndex, guessing would not be needed anymore.