Now that looks like a really useful feature. I only knew of cpio concatenating so far, not that Linux can handle multiple gzip'ed cpios.
My multiboot patchset is on its way upstream. It should be in soon unless something breaks utterly ;-). After that you could write a patch that basically does a split by "," and reads several initrds.
Or is this bug about a feature request because you won't do it? If so, I could take a look at it next time I'm fiddling with the Linux -kernel code.