The founder of the Alliance explicitly forbid other OS developments from other vendors. If you rely on said founder to access alternative OSes you have been warned. Clearly their plans are the opposite of open.
To the point, yes, OEM unlocking is possible now. Nothing prevents Android X+1 from mandating the opposite, just as they requirements on partitions, modules and bootloaders of the members’ releasing with the next Android. Just peek through the number of "devices launching with Android " on, random sample, this page: Generic system images | Android Open Source Project and note the number of “must” uses. I’m pretty sure there is internal OHA documentation with early “must” warnings, we just read it when it is out and some devices already “must’ve” figured it out.