Don't put too much faith in me, I'm figuring this out as I go too!
Do you have serial yet? You'll need it to get anywhere with an unsupported device. First thing is to get serial access to the bootloader, then try tftping an image into it and booting it from RAM. As for which image, well I'd go through the list of RT5350 based devices and pick the one with the closest hardware to your device - as I understand it, the differences are fairly minor, just changes to the configuration files and packages to suit the specific hardware. A build for one RT5350 system is likely to boot ok on a different one with the same amount of RAM. You'll need the .uimage format file for booting from u-boot.
I'll be interested to see if you hit the same LZMA error I get... if you have to experiment with replacing the u-boot, the probability of bricking the device is very high... you'll really need a way to program the flash. I'm betting on the test clip for in-circuit programming, because wingspinner has been able to do this with another RT5350 device, but I havent proved it yet... Should receive the clip in a few days, so I'll let you know how I get on.