A short update
Upon suggestion by a honorable gentleman on the NetBSD-users mailinglist I looked for, found and installed a firmware upgrade (by SuperMicro) for the controller, but with no result. It still acknowledges the tape drive and refuses to give me a st(4) device.
I've ordered a LSI 9211-8i (that does support tape drives) and if it works will configure my system like this:
The 9211 is also based on the SAS2008 chipset, and should therefore work with NetBSD. I looked at some SAS3 cards but they're are still in the ridiculous price range.
Staffan