I just used cpqlocfg.exe 4.21 to send the below script to my iLO2 2.29 and it worked.
<RIBCL VERSION="2.0">
<LOGIN USER_LOGIN="adminname" PASSWORD="password">
<RIB_INFO MODE="write">
<MOD_GLOBAL_SETTINGS>
<IPMI_DCMI_OVER_LAN_ENABLED value="n"/>
</MOD_GLOBAL_SETTINGS>
</RIB_INFO>
</LOGIN>
</RIBCL>
Could it be something with the hponcfg.exe tool you are using?