Yes, we've done a little more testing on this. The latest "trick" I've used is to insulate the tubing and add in some external heating.
First, have a look at our basic tutorial video here, https://www.youtube.com/watch?v=KrR_xOnTUPc
If you want to avoid hardening for longer prints, you have to supply some external heat and insulation. So what I did was make a wool sleeve for the tubing. Next, I got some ironing board fabric, which I used to make a linear series of pockets to place inside the wool sleeve. I put some pocket hand warmers into the pockets, then put that through the wool sleeve. Then I placed the tubing inside that.
You have to be careful with the hand warmers, because the warnings on the package say they can get up to about 69 C, and you don't want your chocolate to go past about 30 C.
Then, you heat up the chocolate as shown in the video (or put the chocolate into a ziploc bag; microwave some water for about 2 minutes; place the bag into the water, submerging the chocolate and leaving the bag open; stir the chocolate until thoroughly melted; cut the corner of the bag and pipe into the cartridge). Assemble the cartridge, tubing, and sleeve and set up the printer.
This rig is cumbersome, but keeps the chocolate melted well for longer prints.