First stage boot loader is not Omap X-Loader. It is totally different bootloader (but only with similar name Nokia X-Loader). X-Loader is signed, so we cannot repleace it.

Second stage boot loader is NOLO (Nokia Omap Loader) which is not signed, so teoretically could be replaced. But U-Boot binary is too big (bigger then NOLO), so we cannot easily relplace it. And we do not have *working* U-Boot port for N900 (as second stage bootloader, which initialize HW) - only port which behave like linux kernel...