I'm talking mainly about the game archive, but there should be some older games around as well. I had some pleasant acommunication with Hagbard, he was helpful. I don't know what open source is supposed to mean in that context. Are we talking about the same kind of database? I'm just talking about the played games.
My main observation is that this thing is really slow.
Advice regarding playing strength: If you are really interested in playing strength, test your engine against other engines. "Other engine" is a pretty flexible term. Test it in "selfplay" against an old version of itself. That already gives a rough estimate whether your last change was reasonable or not. And it's more accurate than me trying to play it.
Besides: I'm definitely no expert for Brandubh. I've just played a game against your engine. I've noticed that the throne doesn't seem to be hostile to white. You see even the rules are new to me.
The second and third game both resulted in stalemates where we both moved back and forth. I stopped after nearly 50 full moves, since the thing is really slow and I didn't want more time watching the "a web page is slowing down your browser".