I can only guess at what you're trying to do
but isn't a simple experiment in order?
From my experience, the source setup in line mode is hard-coded (each source has a specific use) and there isn't much to do about that.
But there might be an exploit waiting to be discovered. I think the state-machine loop is custom for linedrawing, but in the case that B can function as a normal source it should work as in all other blits; the entire line of words pointed to by B is shifted left/right according to DESC. The problem is that BLTSIZE is special for linedrawing, so how does it know how many words to read "as in normal mode"? If you get what I mean.