This is very interesting.
I think the original question left out an important piece in the discussion. What kind of contract, if any, does the "opera" house have? Are they an Opera House only in name, or like us, do they produce all opera and then once in awhile a musical. If it is a non union opera house and not considered as a possible theater for Equity to organize, then there seems no impediment to anyone in any union working there. If, however, they are on a non-Equity touring company circuit, then it is wrong to work there as anything but a picket. Here at NYCO, the Equity performers in all our musicals, like Candide, which we are doing now, sign AGMA contracts and are then stunned at the differences in "protection" of the two unions. On the one hands they can't believe how short the hours are, but on the other they don't always get being called 2 hours before performance for hair and makeup appointments... so it balances.
Anyway, the best advice given is CALL EQUITY! They are the final deciders, but the principal should always be NOT TO WORK for companies who are clearly avoiding equity status to save on pay, work rules, etc... like Big League, etc. They are a plague.
Just my opinion.....
Tom Kelly