Siman 110 mb sk 12 allows an employee to daven mincha with a minyan (assuming that is common practice in that place). Does this also apply if:
1. The employer is a non jew
2. The employer is non observant (and therefore perhaps makpid, see mechaber)
3. If the employee has paid break time
You are asking a good question, but to a degree you included part of the answer in the question. The Mishna Berura 110-12, brings the Magen Avrohom, that this only applies to places where it is the custom, however it is not an automatic rule that the worker may go out for minyan while working. Especially nowadays that we are given an hour lunch break, and coffee breaks during the day. It would be best to ask your boss, if it alright with him. If not, then daven or arrange your lunch break at a time that there is a minyan.