AFAIK there is no legal mandate that says a baseband cannot be flashable. The FCC will not grant certification to a device where the frequencies could be change

AFAIK there is no legal mandate that says a baseband cannot be flashable. The FCC will not grant certification to a device where the frequencies could be changed, but that just means it couldn't be sold directly in the US. You could easily get such a phone from overseas though. I'm not sure how that regulatory structures works on other countries. I'm sure it's mostly similar.