Luckily, preemptive coros don't do that. Everything still runs in one process, one coro at a time. Just passing control from one coro to another can happen implicitly, so blocking code that isn't under your control is not a problem anymore.
Yes, currently you will have to explicitly cede control to the other coroutines regularly. However, Yuval is currently trying to get his patch implementing preemptive coros merged into Coro. Those allow control to be automatically passed to other Coros using different strategies like opcode counting and fixed timeslices. That'd at least solve one of the problems you're describing.
All MooseX::Declare does to not require you to unimport all Moose keywords (and possibly other imports) is
use namespace::clean -except => 'meta';
to remove all previously defined and imported functions (except for the 'meta' method Moose gives you) at the end of compile time. No need to wait for anything.
MooseX::Declare provides similar features. The class keyword only accepts a block with the class definition right now, but i'm also working on implementing "class Foo;" using B::Hooks::OP::Check::StashChange.
I'd be glad if we could have some basic syntax compatibility between MooseX::Declare and your class keyword patch, in case you want to add some kind of options to it so things like
"class Foo extends Bar {
