What you need:

- HxD or some other kind of Hex editor
- FTP on your PS3

Tutorial:

1. Using your FTP server, navigate to /dev_hdd0/home/000000000x (replace x with account you use)/friendim
Transfer your mecomment.dat file to your local disk using your preferred FTP client.

2. Open it with your Hex editor, and choose from this array of hex chars to put in:

EF A2 92 - PS button (untextured)
EF A3 BD - Black Cross (no background)
EF A2 83 - Triangle (untextured)
EF A2 8F - Start button (untextured)

3. Save and upload it to your ps3, overwriting the old one, then exit FTP server and logout of your PS3 user account and log back in.

4. Go to comment and you should see the symbol.