I want to import an ebuild from the main tree for a package whose dependency list varies between platforms, but there is no documentation that describes how to do this.
ABCD suggested in IRC that I handle this by using "kernel_linux? ( ... )" when manually importing ebuilds from the main tree that have Linux-specific dependencies listed that make no sense for the target platform. That was the hint I needed, but I imagine that the documentation could mention this, so I will leave this open.
depends a lot on what's the dependency There is kernel_*, elibc_* and userland_* (see profiles/desc) Since userland in Prefix is always GNU, it'll be close to useless for Prefix use, but the other two can help you sometimes. But in some other cases, introducing a USE-flag and masking it on platforms where it cannot work (or forcing it, if it's necessary) would be a (much) cleaner solution.
Maybe it would be useful to have a link to the Gentoo Development Guide somewhere on the Prefix website. That should help minimize confusion.
Prefix ebuilds no longer are imported or ecopied.