Creating an internal check item with zabbix[proxy,{HOST.HOST},lastaccess] gave me a proper working pattern for my proxytemplate. Refering to the docs it shouldn´t work as the linked footnote says 'macros supported in item key parameters will only work in item types that have interfaces, i.e. they will not work for types “Zabbix agent (active)”, “Calculated” etc.'

https://www.zabbix.com/documentation/2.4/manual/appendix/macros/supported_by_location