The proposed syntax wouldn't work but the concept *IS* useful.
This could work as Riamus suggested, but perhaps with a better name, like $default:
var %x = $default($1, DEFAULT_VALUE)
That wouldn't rely on any grammar modifications, which means it would work everywhere, not just in /set and /var:
//echo -a You are $default(%age, 0) years old.
That said, it's so simple to implement that having a builtin for this seems a little unnecessary. Most people don't need this.
