Forums Scripts & Popups Variable in variable, can this work?

set %away.system.auto.message Auto away after $!duration(%away.system.auto.check,2) of idle
The ! prevents the identifier from being evaluated in the /set command.

Alternatively, add an $eval(,N) and 0 as the N value:
set %away.system.auto.message Auto away after $eval($duration(%away.system.auto.check,2),0) of idle