Not really. If you did get a teaching job in a unionized school district, you'd be paying union dues. So the union wouldn't care how you got there.
But you do bring up a very good point. There's got to be a streamlined way for folks like you to get a state teaching certificate. Take one semester of teaching methods, then off you go.
Perhaps the obstacle here is not the unions, but the college schools of education. The colleges would certainly prefer you take not one semester of teaching methods, but four years.
The certification is essentially a teaching degree. This is not a small task and with the price of college, not a minor expense.