Is there a 'startup' event in DOpus, where I could execute a script?

The OnStartup scripting event is for that. Note that you cannot ensure your script runs before any others which also handle the same event.

There is also, under Preferences / Launching Opus / Startup, a place where you can specify a command to run when Opus starts. (I think that will be run later than scripts, but I'm not sure.)

Don't know, sorry.