I'm still new to Danmakufu scripting, but it seems the SetRenderState function has been replaced by ObjRender_SetBlendType, which needs the object ID and the blend type: BLEND_ALPHA, BLEND_ADD_RGB, BLEND_MULTIPLY or BLEND_SUBTRACT.
From this, it would seem that additive mode does not ignore black backgrounds, which was quite strange in the first place (even if it allowed for "shining" sprites).
Has anyone tried those yet?