I have read these pages:
http://groups.google.com/group/google-excanvas/browse_thread/thread/8762ed1ede73509
https://connect.microsoft.com/IE/feedback/ViewFeedback.aspx?FeedbackID=333905
I’ll be a few extra steps to get everything to apply easily in IE8 (it might mean different function calls for a developer making use of the library), but the raw VML will work correctly in IE8 standards mode. The tests from the above links work well in my virtual machine, and some rendered source code stolen from IE7 and plopped into raw HTML works in IE8 – so long as it is all encapsulated inside a relative-position DIV. I’m not sure why that last part is important, but it appears to be a key ingredient!