Is it possible to blacklist streams from a biome? I have a new biome that is very much not compatible topographically and do not want 200-block-deep nearly-90-degree cliffs and waterfalls throughout.

Not currently but this is on my todo list. Note that the latest version of Streams tends to dig tunnels through mountains instead of the giant canyons it was once infamous for. So until the blacklist is implemented, perhaps it won't look so terrible in your biome after all. :)

That might look better, but my biome is kind of...special.

Oh I see what you mean. :-) You won't get any streams on small floating islands for sure since they need a certain minimum size, not sure about the larger landmasses though. Perhaps they might look OK there. Anyway, biome white/blacklists are on the todo menu.

The issue seems to be the large amount of coastline.

Looking over the scala code (and not understanding all of it), it looks like it will consider any part of the coastline valid for generating a river mouth, and since there is so much coastline in this biome, it will place a Stream in lots of places.

Maybe instead of a white/black list, a biome/chance table, so that most biomes might be 100% (any cell that passes the shoreline test), while other biomes might be 25% (only some of the cells with a shoreline get a stream).

I would highly appreciate a blacklist feature, as I'm using Biomes O' Plenty which implements biomes where a stream doesn't fit, e.g. Mystic Grove, Quagmire or Wasteland.