You don't have to make it a system app. Just disable the ics check. Now, I'm not on miui anymore, but maybe a reboot is needed after a change of the APN values? I have no idea myself.
I just got the information pulled up and tried everything myself.
Hm... I'm pretty sure I tried rebooting as well but I'll double-check. And does it not? Oh, I didn't realize it would work without being a system app, regardless it hadn't actually fixed the MMS issue but I'll try again and make sure I reboot afterwards.