Accessing a WebResource from a .skin file

I have the following entry in a .skin file:

<asp:button runat=server SkinId=btnSubmit ImageUrl="~/Images/
submit.gif" />

But I want to make the image an embedded resource in a different
assembly, using WebResource.axd.

How can I do this?

Expression Builders don't work in skins
<%= WebResource("MyAssembly.skin.gif") %> won't work in a skin (<%=
doesn't work %>)

- there's no way of running any code, the necessary
GetWebResourceUrl() call cannot be executed.

There doesn't appear to be any way of accessing an embedded
WebResource from a skin. Is this correct?