Thanks for the excellent writeup! For u-boot, I find this a bit surprizing as I thought Overo was well supported in upstream u-boot; in any case, please do file a bug against u-boot-linaro on this. It's quite likely patches not yet upstream to u-boot which we might find in Steve Sakoman's tree(s).
For linux, it could either be a missing patch or a config option or a kernel cmdline option, but I guess you tried the latter already; I don't know where to start searching for this, but it's certainly also worth a bug against linux-linaro.
Cheers,