Inline Crossovers

Will they work?
Will they sound good?
I doubt it.
Given the tiny amount of space inside the barrel of those things, they're probably using some non-audiophile parts. Really gross stuff like ceramic or electrolytic caps. (I don't have time to read the fine print--they may say what they use in the text...)
Overall, you can use passive components to build a passive, low level crossover. Just be aware that you'll have insertion loss and possibly impedance matching problems.