Fantastic!!! Thank you so much for your help!!!
I had already tried putting the fonts into editor.css but it looks like $contentcss is using the editor.css in the base theme for the editor rather than the current theme.
I did this in /lib/editor/tinymce/lib.php in function get_init_params()
$contentcss .= ', /theme/mytheme/style/fonts.css';
$fontselectlist .= ';Aka Dora=akaDora';
And in /theme/mytheme/style/fonts.css
Purged the caches and it works! :D
I still need the @font-face in /theme/mytheme/style/custom.css too so the content is displayed correctly but I'm just thankful to get it working ;)