Does "バルス", which is popular in tweeting during Laputa, have any meaning or etymology, or rationale for being in katakana?
If you recall from the movie, the word "BALS," is the Laputian incantation the collapses Laputa. In the Laputian language it means "to close."
On the night of August 2 (2013), during the "Friday Roadshow" airing of the Japanese anime film Laputa: Castle in the Sky on Nippon Television Network, Japanese netizens renewed the world record of Tweets Per Second (TPS) at 143,199 TPS by all (simultaneously) tweeting the magic spell "BALS" at the same time as the protagonists Princess Sheeta and Pazu read the spell together during the film. In the story, the magic spell "BALS", meaning "close" in the language of Laputa, is a spell of destruction that destroys the city of Laputa. The spell is said to have derived from the Turkish word "barış," meaning "peace."
The amount sheer amount of tweets wound up overburdening Twitter's servers, resulting in the appearance of the the well-known "Twitter Whale." The overwhelming popularity and notoriety of the tag (it's said that "BALS" was the ironic cause of the sever collapse), it stayed and eventually became a fixture in on the Japanese side of Twitter due to this event.
Katakana are used for words that are not actually japanese.
Since Balse is a word in Laputian language it has to be written in katakana.