H323 involves set of protocol suits. Mainly h225 & h245. Certain packets of these protcol carry the IP address in their payload as per the protocol requirements. ASN1C library is used & modified to parse these packets and replace the internal IP that they carry in payload with the NAT IP.
More details here : https://arraynetworks.atlassian.net/browse/AE-1179

Cli added:
AN#sh h323

H323 natting is enabled.
AN#

AN(config)#h323 ?

nat                 enable/disable natting
AN(config)#h323 nat ?

disable             Disable H225 natting

enable              Enable H323 natting
AN(config)#h323 nat enable
AN(config)#