Guys, it looks like i found an elegant solution.

Please check the new-finalization-auto.2.cs, which i uploaded to
http://bugs.squeak.org/view.php?id=7473.

It adds just a few things:

initTestPair
	TestItem := WeakFinalizerItem new list: TestList object: Object new.

checkTestPair
	HasNewFinalization := TestList swapWithNil notNil.

These two method is then sent from within WeakArray's finalization process:

finalizationProcess
	[true] whileTrue:
		[ WeakFinalizationRegistry initTestPair.
		FinalizationSemaphore wait.
		FinalizationLock critical:
			[
			WeakFinalizationRegistry checkTestPair.
			FinalizationDependents do:
				[:weakDependent |
				weakDependent ifNotNil:
					[weakDependent finalizeValues]]]
			ifError:
			[:msg :rcvr | rcvr error: msg].
		].

And so, at each GC cycle, a new registry it knows exactly if it
allowed to do a shortcut, or not.

And if not, then it behaves in same way as old registry - it starts
scanning the whole dictionary to
clean graveyard.

So, in summary: running on older VMs , a WeakFinalizationRegistry will
behave exactly as old WeakRegistry.
While on VMs, which supporting new finalization, it will do a shortcut.

This is at cost of extra 2 objects, allocated per GC cycle, to detect
if new finalization is supported.

P.S. oops, actually it could be just one object.. just need to change
two methods at WeakFinalizationRegistry class side:

initialize
   TestList := WeakFinalizationList new.
   TestItem := WeakFinalizerItem new.

initTestPair
   TestList swapWithNil. "make sure list is empty"
   TestItem list: TestList object: Object new.

-- 
Best regards,
Igor Stasenko AKA sig.