Yep, used this method in our last email send and it worked perfectly!
I also use the subject line assistant: Subject Line Assistant (I thought I got this from Sanford's post but maybe not)
You can copy an emoji from just about anywhere and paste what you want (emoji included) for the subject line into that assistant to get the code you'll need to put into Marketo!
paste that into SLA and you get
It works well! Still, of course, test, but I haven't seen many issues since using it!
Good places to get emojis: