I could find and working on a carry system that would allow to carry the gun in two pieces, which will reduce the overall packing length.
I don't think you can do that. The headspace will need to be checked everytime you remove the barrel on one of those from what I've read. (I had an HR years ago but I was only a teenaged lad.)
This is an interesting option. http://www.hr1871.com/Firearms/Shotguns/tamer.aspx 20" barrel instead of 18, but that's not much to deal with.
RR, I still think a 20g pump youth is the best model for you (given your preferences stated in the other theread) because the barrel is easily removed giving you two pieces that would easily store in a large pack. Mossberg youth is only 5 1/4 lbs.