Hi Grant,
Yes, currently seL4 only supports 32-bit guests. This is something we are working on ameliorating in the future, keep an eye on our upcoming releases.
I have an odroid-xu4 on my desk at the moment, as it will soon become a supported platform, but I haven't started the port yet. However I suspect it will only require minor changes to the exynos5410 (odroid-xu) which we already have arm hyp support for.
The porti s scheduled to occur by the end of the year at the moment.
I don't think its very different from the xu4, and might work with only a few tweaks - they both have cortex-a15's. There's minimal documentation available from the odroid-exynos platforms unless you sign an NDA with Samsung, so I've only got the device tree in the hardkernel linux branch to go on.
Cheers,
Anna.