I've posted livestream announcements there before (only for large livestream events like our Valentine's Day Stream, Q&A's, etc.) and got decent reception, but I can assume it's because it's something they could take part in.
Honestly, I would suggest making a separate series where you post bonus content, or just tacking it on to the end of episodes. I post to Webtoons once a week with 10-15 pages on average and sometimes that's still not enough for 'em
I feel like Tapastic is sort of easier to post bonus content to because it has a lot more opportunities for communication than LINE. You can't make wall posts or anything on LINE, just comments, so it kind of becomes a game of "to post or not to post" there.