It IS usable, but there are a few bugs here and there, so I could release it now really. A lot of things are unfinished however, and missing, this being such a huge mod. It will be released as v1.0.0 when everything has been done.
Maybe put a pre-realease so we can help you find bugs that you can fix
Maybe since there will be sap trees in the BL now, I'll list a few uses for sap in the real world.
1. Sap from many trees historically was processed, into turpentine etc. After that, it could then be used for many things. Some people used it as a more efficient lamp/lantern fuel in place of the whale and coal oil used in those times. Turpentine was also used as a solvent to clean things. Like dissolving tar.
2. Sap has been used in multiple cultures for medicinal purposes. One example: it would be taken internally (pine sap mainly) in order to rid the body of parasites in the digestive track.
4. Industrialcraft doesn't have to be the only mod with resin/sap converted into rubber for uses. But then again maybe rubber doesn't fit in with the BL.
5. Also, sap was used for trappings bugs and stuff. Fossilised sap as amber is a good example.