[mfyvie]

Finally managed to solve this, and thought I'd share this here for anybody else who tries the same thing.

After several hours of sifting through tutorials, I found this one about adding bitfields.

I read through it and eventually figured out that I could adapt it to non-bitfield options. Therefore all I was missing was this code:

if (!isset($this->validfields['mloginexempt']))
{ 
    $this->validfields['mloginexempt'] = array(TYPE_BOOLEAN, REQ_NO);
}

Which I inserted at the userdata_data hook.

Yes, I know, I probably should have used a bitfield for this, but I'm just adding this on at the end. I'll know better for next time