He gave an update on the support at LCA2013 in Australia this week while he's just wrote on his blog a brief summary and also made his slides available.
Overall, they had to re-architect their pre-bootloader because it wouldn't work with the simple Gummiboot EFI boot-loader. It turns out that it wouldn't work due to differences in how Gummiboot boots its kernels compared to shim and other alternatives.
In the end they got their Linux Foundation Pre-BootLoader working with Gummiboot and it was demoed this week at the Australia Linux Conference. A separate blog post to come by Bottomley will go over how the new architecture of their pre boot-loader works.