You could try tumble drying them on air fluff (no heat) for a few minutes. This will help to separate and fluff the towel loops. Then hang them on the line. When they are nearly dry, stick them in the dryer again, this time on heated dry, to finish them.
They may not be as soft as completely machine dried (there's something about heated drying of wet towels that makes them softest) but they will be softer than if 100% line dried.
Another, non dryer method, could be to shake out the towels thoroughly and vigorously before and after line drying. Similar tactic as the air fluffing, just more work for you. I haven't tried the shake technique, myself, but it might work.
Me, I tried line drying for some years using the above air fluff technique. But eventually I got tired of moving the towels back and forth between dryer and line, and now I simply spin them as fast as possible and then machine dry them all the way. The cost in energy is fairly low. Sheets, blankets, bedspreads, comforters, mattress pads - these all get line dried, weather permitting. They don't need to be soft and I kind of like the starched effect that line drying can give to all cotton sheets. The synthetic bedspread (polyester) is soft no matter how it's dried.
