I'm calling kexec as so:
# kexec --load-hardboot --mem-min=0x50000000 zImage --initrd=initrd.img
so I think I'm already doing that. My test kernel is LeanKernel which I unpacked the boot.img into zImage, initrd.img, and dt.img. First thing is I most likely need to pass the correct command line which I haven't messed with much, second is that I probably need to pass dt.img as well. I'm using a new version of kexec-tools with dtb support but was unable to use --dtb dt.img (had a buffer overflow error, may need to increase a limit somewhere).
Samsung Galaxy Note 3 SM-900T 32GB
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HP TouchPad 32GB
Archos 43 Internet Tablet - 16GB + 32GB mSD
Dell Axim X50v - 128MB + 16GB CF