I suggest that you ask this question on the Haiku development mailing list, which is where the Haiku devs frequent more than on this forum, so you are more likely to get an answer. You can subscribe here:
You could also create a tiny app which can print the BMessages it receives, and run this in the terminal. Now you can let your add-on send the message to your custom app instead of printing them. Et voila, messages from your add-on in the terminal stdout.
If you have another solution from the mailinglist, maybe it is a good idea to post it here.
Yeah, I figured I could resend the BMessages to another App, I was just lazy : ) I will probably do that next time I sit down and play with this again.
However, I did get this recommendation from Ingo on the Haiku developers mailing list:
The easiest way is to redirect the output of the Bootscript
(/boot/beos/system/boot/), i.e. replace