More to the point, it's not like DirectX is some kind of uberframework offering the most convenient and complete API for game development. In some respects it's actually rather clumsy and doesn't really provide anything you couldn't get on non-windows platforms.
Depending on how LFS is internally designed, it actually shouldn't be that hard to port it for *NIX platforms. Let's see how that could be done.
3D Engine: Arguably the most time consuming part, however, OpenGL is no worse that Direct3D and the rather simple nature of LFS 3D engine would make the switch quite possible in a reasonable amount of time.
Audio: Just use ALSA if you want to focus on Linux only or OSS to support all sorts of *NIX OSes.
Input/FF: Linux supports a lot of gaming devices natively and programmer can access them through /dev/input/jsX. Same goes for FF.
Networking: LFS uses WinSock (Windows implementation of network sockets) which is very similar to socket implementations found in most *NIX systems, so a very little work would be needed on this ground.
I'm not advocating a *NIX port of LFS here, it's not worth the extra work and LFS runs just fine with WINE, plus maintaining a complex multiplatform source would be rather stressful for just one dev. I just wanted to point out that people are often exaggerating the difficulty of porting. A native dedicated server would be very nice though...