Here is my system with 18" up to 800hz. Yes, I did think of the 4435 when putting this system together. I used to own 4435's and the sound is somewhat similar. The goal was to have a highly effecient system that was relatively simple.
There are probably some negatives to using the 2241 so high, but there are some positives to the system. I like the 2-way sound, the woofer and driver are very close and sound cohesive and the distortion of the 2" driver is very low. The system might be improved by using a non-JBL 18 that is a little less damped. I have heard a TAD TL-1801 18" woofer in a similar system that sounded very nice.
I am not a speaker designer or expert by any means. I wish someone with the expertise would give this a try. I think someone with the right knowledge and test gear could develop a very nice 18" two way system.