XCI loaders are hard to write as the game cart slot was believed to be the main target for piracy. The LOTUS3 chip is still unbroken to this day and not much information can be found about it except:
https://switchbrew.org/wiki/Gamecard_ASIC. SX easy way of implementing their XCI loader is highly illegal and could never be done like this in an open source project. In the end modifying FS would probably be the easiest way to get XCI loading to work but in that case adding NSZ support to CFW and maybe even NSZ loading would be easier.
In addition there's also a moral issue with XCI loaders. Assuming somebody owns a cartage, most people don’t see a good reason why he should mount his XCI dump over just inserting his cartage. This makes the majority considering it as piracy tool and so hurting the developer’s reputation a lot. Because of this support from other developers will be extremely limited as they refuse to help developing a piracy tool. Writing an XCI loader completely without any help seems unfeasible - at least for me. Please also keep in mind I developed nsZip with legitimate dumps and emulators in mind and don't want my work to be associated with piracy in any way.
Anyways if there will ever appear an open source XCI loader I will implement XCZ support for sure.