I doubt it is possible to provide for, even only *Plug/BeagleBoard a kernel rpms. At least *Plugs use a cut down version of uboot, which only (I did not see anything else) supports kernel flashing from tftp.
I think it is only possible to achieve the goal of multiple platforms with setup howto for each pseudoplatform and a base rootfs with bash, tty, ssh, and other "basic" components plus maybe some package groups like "guruplug", "sheevaplug" to enable full support of each devices functionality.