I can use the lava sigil from Blood magic to fill up a smeltery from Tinkers construct (click on drain just with a bucket). This does not work for tanks from Neotech.

Is this a bug or just a missing feature?

Try the latest build from Jenkins please. I have changed how things are handled to the new system http://teambrmodding.com:8080/job/NeoTech/184/ with http://teambrmodding.com:8080/job/Bookshelf%20API%201.8.9/42/

I will have to look into how he is doing it.