Currently, it is not supported to use global macros such as {HOST.HOST} inside trigger expressions. I wanted to create a trigger alerting if the "agent.hostname" is different than the host name configured for the corresponding host inside Zabbix, as this will cause issues receiving results of active items. Unfortunately, this is not possible:

{Template Module Zabbix agent:agent.hostname.last()}<>"{HOST.HOST}"

Also, something like this (for earlier versions of Zabbix when string compare was not supported yet inside Trigger expressions) did also not work:

{Template Module Zabbix agent:agent.hostname.str("{HOST.HOST}")}=0

I think it would be very helpful to be able to do this.