Hmm? The onus is, and has always been, with whomever is proposing the new interfaces.
The liaison might get some flak from whomever is proposing the new interfaces and being told to go back to the drawing board. But the liaison will be in a position of power in that case (as long as he is someone that is respected by Linus for his good taste in ABIs/syscalls).
The way these things have worked in LKML lately is that you either do _something_ productive when people decide to oppose your new syscall/ABI, even if it happens at the very last moment, or you are at a reasonable risk of all your effort being declared "not for mainline".
But yes, it HAS to be someone respected by both sides, or someone who is respected in LKML (he has to have power to reject bad interfaces since day one), and who has a large potential to become respected within the glibc community.