Not if you weren't hired as an RN.
You are to work within the scope of the position in which you are hired.
Sorry about your difficulty. But, I believe going back for an MA will be a waste of your time and money. I know you are getting desperate, but I would suggest that rather than filling out online applications, you find out who the managers of given units are and contact them directly. Continue to contact them and emphasize how badly you want to work for their unit- not the hospital, not the specialty- but their unit.