If someone is interested, it looks like sipwise has made a working patch (thanks for pointing out linuxmaniac): https://github.com/sipwise/kamailio/commit/3890714cf2011ec9b137d4068ca33f226...
After some trial and error, I managed to get this XMLRPC request working:
curl -H "Content-Type: text/xml" -X POST -d '<?xml version="1.0"?> <methodCall> <methodName>pua.publish</methodName> <params> <param><value><string>sip:544460@KAMAILIOFQDN</string></value></param> <param><value><string>3600</string></value></param> <param><value><string>as-feature-event</string></value></param> <param><value><string>application/x-as-feature-event+xml</string></value></param> <param><value><string>.</string></value></param> <param><value><string>.</string></value></param> <param><value><string>sip:KAMAILIOFQDN:5061;transport=tls</string></value></param> <param><value><string>P-Flags:0</string></value></param> <param><value><string><![CDATA[<?xml version="1.0" encoding="ISO-8859-1"?><ForwardingEvent><device><notKnown/></device><forwardingType>forwardImmediate</forwardingType><forwardStatus>true</forwardStatus><forwardTo>477</forwardTo></ForwardingEvent>]]></string></value></param> </params></methodCall>' https://KAMAILIOFQDN:5061/RPC/
Cheers, Dries