You have to run the ActiveX control in a separate 32-bit process. That's going to be difficult, it would have its own window that isn't going to be part of the UI of your 64-bit process. Although it is expressly forbidden by the SDK docs, you can try to take advantage of the Windows 3 appcompat built into the SetParent() API function. It might work.
You'll have lots of additional trouble, communicating between processes is tricky enough (you'll need Remoting or WCF), the hard part is dealing with exceptions. One process bombing with the other one surviving and never noticing that something is wrong is not going to be pretty.
Perhaps the Platform Target option starts sounding attractive?
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