For x86(_64), OpenWrt generates disk images, similar to the situation for other targets, ISOs are currently not provided (and would require quite some changes). It's not really meant to multi-boot either, as this doesn't really make sense for a typical router (which usually runs 24/7), but this would be easier to get working manually.
If you're interested in working on these features, patches would probably be welcome (ISO images might be beyond the scope though, as these are readonly and can't hold state, so they'd need some installer facilities, which would impose a lot of structural changes for no gain).