Allow IF EXISTS for UPDATE statements

While creating a hands-on exercice for Cassandra, I was facing a quite annoying limitation.

Let's take this table:

CREATE TABLE killrchat.chat_rooms(		
	room_name text,
	creation_date timestamp,
	banner text,
	creator text,
	participants set<text>,
PRIMARY KEY(room_name));

Upon a new participant joining the room, to be concurrency-proof (avoiding mutating the participants set if the room is deleted concurrently), I would like to issue this query:

 UPDATE chat_rooms SET participants = participants + {'johnny'} WHERE room_name = 'games' IF EXISTS;

Unfortunately the clause IF EXISTS is not allowed for UPDATE statements. Similarly I tried

 UPDATE chat_rooms SET participants = participants + {'johnny'} WHERE room_name = 'games' IF room_name='games';

It doesn't work either, it is not allowed to use one column of the primary key as condition column for LWT (why ? mystery).

So far, the only work-around I found is:

 UPDATE chat_rooms SET participants = participants + {'johnny'} WHERE room_name = 'games' IF name='games';

I added an extra column called name which is just the duplicate of the partition key room_name. It does work but is not very elegant.

I believe there are legit use cases for UPDATE ... IF EXISTS;