With a tight deadline, waiting for the WMV fix might not be an option for you. But, if you have Sony Vegas, Sony Movie Studio (Most versions) or a video editor that allows you to import a sequence of stills, you can ultimately render to any video format.
The Sony video editors allow you to set a default length for each individual image as it is imported. While I don't know precisely the expected timing that VideoScribe assumes, I've found that something between .033s (30FPS) and .066s (15FPS) works. I ended up using .05 seconds.
If you use a sequence of PNGs, you will have to add a background under the sequence track. Otherwise, the background defaults to black.
In the end, this may be the best way to create the most precise and cleanest videos because it is possible to refine timing in Sony at an individual frame level. It's certainly a great option for creating overlays that have to match the timing of existing videos.
This a great product and seems to offer almost unlimited potential for those of use creating training content for the web. I'm sure they will sort out the WMV issues; but, in the meantime where there is a will they have provided the way! :)
Thanks for sharing Tom, just to advise that PNG sequence rendering is a Pro feature.