You might check out amanda, which does dumps to holding disk and then
has a taper program which streams to the tape drive.  I don't use it
with DLT, but it keeps a dds3 at ~1 MB/s streaming.

I suspect that really you need a tape-streaming program that buffers
things from tar and then writes to tape, so that tar can be running
while the tape is writing.  This probably requires something that acts
like threads.
-- 
        Greg Troxel <gdt@ir.bbn.com>