Some clever reinforcement in the luff area would do the trick. It's just a matter of making it strong enough. But now you have me wondering.. I have not buildt that many sails so there is a lot of room for learning (and wrong opinions). I tried finding some good photos of the Bim18HT main, but none came up.
Hopefully somebody have the correct answers to share.

I take it the Bim18HT dont have a halyard for the main, since you use the plastic tube? If you have a halyard, a luff rope should be just as good. Especially so since the luff probably already is strong enough to handle all downhaul loads. If you dont have a halyard, I suppose you need the plastic tube to raise the main. Unless the tube can be repaired (what plastic material would crack and then permit repairs?), it sounds like it have to be replaced. Again, replacing a luff rope is not a large job, I dont see why using a plastic tube instead of the rope should make it harder?