Can someone please suggest what should be a value of in the seagull script so that Wireshark correctly decodes MNC-030 and MCC-234?
It sounds like you're wanting to send the PLMN in the proper format in a Diameter message such as a ULR. The value has to be entered using the method defined by ETSI. In your case it would be 0x320430. See the example code below.
<command name="ULR">
<avp name="Session-Id" value="value_is_replaced"></avp>
<avp name="Vendor-Specific-Application-Id">
<avp name="Vendor-Id" value="10415"></avp>
<avp name="Auth-Application-Id" value="16777251"></avp>
</avp>
<avp name="Auth-Session-State" value="1"></avp>
<avp name="Origin-Realm" value="origin.com"></avp>
<avp name="Origin-Host" value="seagull.origin.com"></avp>
<avp name="Destination-Realm" value="destination.com"></avp>
<avp name="User-Name" value="234030123456789"></avp>
<avp name="RAT-Type" value="1004"></avp>
<avp name="ULR-Flags" value="34"></avp>
<avp name="Visited-PLMN-Id" value="0x320430"></avp>
</command>
The ordering of the PLMN digits is outlined in TS 129 272, section 7.3.9.