I added a test in, though I didn't actually check it no further than making sure the C++ generates the correct WinMD (since my machine is very limited in spec and I don't want to slam it).
I did manage to test the source generator using another project by casting a XAML resource into a Style using cast expressions and that did work.

I don't think this can be easily handled in a source generator.
The type cast can happen in anywhere especially now we have pattern matching, for example,

if (obj is { A: Foo { B: Bar(_, Baz { C: [WinRTType x, ..] }) })
{
    // use x here
}

switch (obj)
{
    case  { A: Foo { B: Bar(_, Baz { C: [WinRTType x, var y] }) } when y is AnotherWinRTType:
        // use x and y here
        break;
}

We should instead doing this at IL level so that we only need to look for isinst.