After some delay I have finally committed sane-backends-1.0.16 into portage. This version includes quite a few fixes which were added as patches to Gentoo’s sane-backends before, so I could drop those. Which is not included is the patch for Brother MFC scanners. The Gentoo patch for these scanners was taken from some rather smelly sourceforge project, but (after some work on it) it was reported to work well.
I currently don’t have much spare time and didn’t yet manage to adapt this patch to current sane-backends, so if you use this particular kind of scanner, you should better stick with the old version until I find time for it or someone else manages to do this.
Another issue: Since Diego’s patch to the sane-backends ebuild was applied, you can use $SANE_BACKENDS to pick which backends should be built. This seems to cause problems for a few users which I wasn’t able to reproduce or solve so far. Please, if you have a little spare time, try and see if you can find the reason for this trouble.