Since RCLI is read-only again there is no way to enable SNMP remotely. However the following "unsupported" mode still works:
1. Edit /etc/vmware/snmp.xml
2. /sbin/services.sh restart
3. Verify & test your settings with vicfg-snmp
If you're using the free version of ESXi and specifically U4, then no there is no other supported method of enabling SNMP or performing any other write operations. As you've already verified, going into the unsupported SSH busybox console is one method or you can use the VI Client to enable SNMP.
VMware vExpert 2009