I don't know if it can be considered as full time job, but you can relativelly easy reach 60 hours in a week. My wife was working for several modeling and actra agencies for some time. It's not unusual when day of filming is reaching 12 hours or even more. Saturday/Sunday work is also not unusual. In most cases the clients of these agencies (it maight be film studios, corporations or big retailers which are filming/shooting commercials) are asking for work permit. They also providing T4, even just for one day of work. So its not like you can hide this type of work from CRA/CIC.
But I agree, it's virtually impossible to get lmia for this type of job as modeling agency is not an employer. And long term contracts is not something very common in this industry.