Mike, you always seem to choose the most complex and torturous route to a solution. LCDs are slow - both with the interface and the rate at with the LCD crystals will change. Also, you've got a human on the other end reading it - they are slow (compared to the processor).

If you're worried about the LCD tying up precious cpu time, then you're not thinking the problem through. As for using completion interrupts from the lcd - I'd say it would take a little bit of extra logic to implement this, but why? One solution I choose for not waiting for the LCD was to use a 1mS tick. Each mS I would write a character to the LCD. I wouldn't have to wait, since 1mS is more than enough for the LCD controller to do its stuff. So for a 16*2 display, a full update would take 32mS -or about 31 updates per second. The LCD crystals to change take around 100mS (depends on temperature) and a update of 10 times per second is usually faster than most humans can read.