what rule applies when a statement in the news for eg. reads as follows;
"Chemical Ali to be hanged". I.e. hanged is the past tense for an event that is to occur in the future
Just like it is - and as you noted, it's a news headline way to write things.
Statue To Be Unveiled in Park on Sunday
Committee's Selection To Be Announced at Next Town Meeting
... to be [participle]
[a writer, not a teacher]