Summary of e-mail from sythenast from http://franz47root.wordpress.com/2007/07/27/linux-on-nokia-6630, 2009-06-18 14:43

He had been exchanging e-mails with Vovan888 (ubootloader creator, together with SERRGE) years ago.

ubootloader’s leave error = -2 indicates a check-sum mismatch. I have obtained a modified ubootloader from him, which surpasses this error and goes one step ahead:

ending with Еrror -21 while loading logical device (-21=KErrAccessDenied – because uboot tries to load the LDD, Logical Device Driver, into the protected memory area)
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Bph&co on forum.gsmhosting.com, 2009-06-17 16:39
There is no known hack to execute code on BB5 generation in normal mode. And if there is one, could be used for much cooler stuff like SL3 unlocking or IMEI change (not that Linux on Nokia is not cool idea though).

You can execute code in loader mode via trick, so theoretically, if you solder small MCU in the phone, you can trick the phone to boot in loader mode, load your own pre-loader, then re-init secure rom, MMU etc to pass execution to Linux image in flash.