I have been looking for a way to Flash a Kernel without having to use a Computer so you can do things more on-the-fly. Which is mostly the basis of my How To. Easy and Quick with a little knowledge needed as possible.
I am thinking you could download the kernel straight to your phone and then install as a ROM via ROM manager....ultimately, the ROM manager just does what you explain in your kernel section except it does all that manual stuff for you.....I THINK!!!!!?????
I have changed kernels, ROMs, and themes on the fly with .zip files that were already on my SD card. No reason (that I can figure) why you couldn't just do as flyjbaker is stating and download a kernel straight to your phone, then flash via ROM manager OR by booting in to recovery and flashing manually. The only thing that I probably would recommend is to get any of your .zip files moved to the root of your SD card, rather than in a folder, but that can easily be accomplished with Astro.