An exciting development today. We finally have copy/paste of ink strokes between InkSeine and OneNote doing something reasonable. So this is looking more likely to get into our initial release, provided we don't find any severe landmines with it during further testing!
This three-part capture shows the original content in InkSeine, the strokes appearing "selected" after doing a Paste in OneNote, and then the final look of the strokes in OneNote after tapping to clear the selection.

The fidelity of our stroke thickness conversion isn't perfect, but it's close enough that it shouldn't matter for most practical uses.
The only catch right now is that if you select ink strokes and bitmaps or other stuff from InkSeine, we only paste the ink strokes into OneNote so that you can get at their inky goodness. However, it is a simple matter to grab a screen capture of your ink strokes mixed in with bitmaps if that is what you want instead.
My only remaining wish is that 'FeralBoy' will forgive me for appropriating his signature "inky goodness" catchphrase [:-)]
Update: Since my original post, we've discovered that on Windows XP, Microsoft.Ink.dll under the Common Language Runtime (CLR) 2.0 has a bug that prevents inky goodness from happening by default. Since InkSeine is 100% written in managed code, this issue affects us.
But there is good news! The issue is described in this knowledge base article and has a patch available. Copying and pasting ink between InkSeine and other applications will work as long as you install the update and reboot your Windows XP tablet.
I've also been able to verify that ink strokes can be copied and pasted between InkSeine and Windows Journal. However, Windows Journal highlighter strokes cannot yet be pasted into InkSeine correctly. Nevermind, we've fixed that too!