Henry, I believe the spec is currently missing a bunch of explanatory notes for this tag. The Icon tag should be described like this:
The Icon element has a mode and a type attribute. The type attribute is
simply the mime type of the image. The mode attribute only has one
valid value, "base64". If the value is base64 and the inner content of
the image tag must be a base64-encoded image. If the mode is not set
then the content of the tag is a url pointing to the image.
This explanation means that we already support what you need, so we should be good.
For this issue we should simply make sure that this explanation gets into the docs.
I'm considering this thread closed unless anybody disagrees.
Thanks!
- Cassie