Okay, I figured it out thanks to the interwebs and Google.
For future reference, if anybody has this problem, the fix is :
after getting your embedded font as a stream, and before calling AddMemoryFont,
you have to call AddFontMemResourceEx !
(Not available in C# so you have to import it :
[DllImport("gdi32.dll")]
private static extern IntPtr AddFontMemResourceEx(IntPtr pbFont, uint cbFont, IntPtr pdv, [In] ref uint pcFonts);
and then :
//create an unsafe memory block for the data
System.IntPtr data = Marshal.AllocCoTaskMem((int)fontStream.Length);
//create a buffer to read in to
Byte[] fontData = new Byte[fontStream.Length];
//fetch the font program from the resource
fontStream.Read(fontData, 0, (int)fontStream.Length);
//copy the bytes to the unsafe memory block
Marshal.Copy(fontData, 0, data, (int)fontStream.Length);
// We HAVE to do this to register the font to the system (Weird .NET bug !)
uint cFonts = 0;
AddFontMemResourceEx(data, (uint)fontData.Length, IntPtr.Zero, ref cFonts);
//pass the font to the font collection
mFontCollection.AddMemoryFont(data, (int)fontStream.Length);
//close the resource stream
fontStream.Close();
//free the unsafe memory
Marshal.FreeCoTaskMem(data);
And presto, you'll be able to use the font.
Without the AddFontMemResourceEx it wont work.