I’m not sure either. There have been discussions about that sort of thing in the past, but I’m still not sure.
From 2005:
–Foreign workers to stage rally on Sunday (this one doesn’t seem to discuss whether foreigner participation is allowed)
From 2008:
–Are foreigners allowed to participate in political campaigns - #21 by maoman
From 2013:
–Public protests and foreigners? - #4 by Mucha_Man
From 2014:
From 2017:
–How can somebody join a political party in Taiwan?
From 2018:
–Is it legal for foreigners to help campaign?
From 2021:
–What can be done to help our SEA friends currently suffering - #20 by Marco
From 2023:
–How vocal are foreigners allowed to be when it comes to TW politics? (2023+ edition)
There may have been other discussions, but this is what I’ve been able to find at the moment.