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У меня сохранилась дока по MQ 5.2 - Application Programming Reference, процитирую:

ClientConnPtr (MQPTR)
Address of MQCD structure for client connection.

ClientConnOffset and ClientConnPtr are used only when the application issuing the MQCONNX call is running as an MQ client. By specifying one or other of these fields, the application can control the definition of the client connection channel by providing an MQCD channel definition structure that contains the values required.

If the application is running as an MQ client but the application does not provide an MQCD structure, the MQSERVER environment variable is used to select the channel definition. If MQSERVER is not set, the client channel table is used.

If the application is not running as an MQ client, ClientConnOffset and ClientConnPtr are ignored.

If the application provides an MQCD structure, the fields listed below must be set to the values required; other fields in MQCD are ignored. Character strings can be padded with blanks to the length of the field, or terminated by a null character. Refer to the MQSeries Intercommunication book for more information about the fields in the MQCD structure.
Field in MQCD Value
ChannelName Channel name.
Version Structure version number. Must not be less than MQCD_VERSION_6.
TransportType Any supported transport type.
ModeName LU 6.2 mode name.
TpName LU 6.2 transaction program name.
SecurityExit Name of channel security exit.
SendExit Name of channel send exit.
ReceiveExit Name of channel receive exit.
MaxMsgLength Maximum length in bytes of messages that can be sent over the client connection channel.
SecurityUserData User data for security exit.
SendUserData User data for send exit.
ReceiveUserData User data for receive exit.
UserIdentifier User identifier to be used to establish an LU 6.2 session.
Password Password to be used to establish an LU 6.2 session.
ConnectionName Connection name.
HeartbeatInterval Time in seconds between heartbeat flows.
StrucLength Length of the MQCD structure.
ExitNameLength Length of exit names addressed by SendExitPtr and ReceiveExitPtr. Must be greater than zero if SendExitPtr or ReceiveExitPtr is set to a value that is not the null pointer.
ExitDataLength Length of exit data addressed by SendUserDataPtr and ReceiveUserDataPtr. Must be greater than zero if SendUserDataPtr or ReceiveUserDataPtr is set to a value that is not the null pointer.
SendExitsDefined Number of send exits addressed by SendExitPtr. If zero, SendExit and SendUserData provide the exit name and data. If greater than zero, SendExitPtr and SendUserDataPtr provide the exit names and data, and SendExit and SendUserData must be blank.
ReceiveExitsDefined Number of receive exits addressed by ReceiveExitPtr. If zero, ReceiveExit and ReceiveUserData provide the exit name and data. If greater than zero, ReceiveExitPtr and ReceiveUserDataPtr provide the exit names and data, and ReceiveExit and ReceiveUserData must be blank.
SendExitPtr Address of name of first send exit.
SendUserDataPtr Address of data for first send exit.
ReceiveExitPtr Address of name of first receive exit.
ReceiveUserDataPtr Address of data for first receive exit.
LongRemoteUserIdLength Length of long remote user identifier.
LongRemoteUserIdPtr Address of long remote user identifier.
RemoteSecurityId Remote security identifier.


И еще приведу цитату из MQ 6.0 Infocenter -
http://publib.boulder.ibm.com/infocenter/wmqv6/v6r0/index.jsp?topic=/com.ibm.mq.csqzaf.doc/cs12350_.htm


MQSERVER

This is used to define a minimal channel. It cannot be used to define an SSL channel, or a channel with channel exits. It specifies the location of the WebSphere MQ server and the communication method to be used. Note that ConnectionName must be a fully-qualified network name. The ChannelName cannot contain the forward slash (⁄) character because it is used to separate the channel name, transport type, and connection name. When the MQSERVER environment variable is used to define a client channel a maximum message length (MAXMSGL) of 4 MB is used, so larger messages cannot flow across this channel. For larger messages a client-connection channel must be defined using DEFINE CHANNEL, on the server, with MAXMSGL set to a larger figure, or using an MQCONNX call with MaxMsgLength set in the MQCD referred to in the MQCNO structure .

То есть вызов MQCONNX дает то, что нужно. Но я не уверен, что это сработает в 5.3