Why as a matter of fact....I just did this exact thing a couple months ago. The show I did in DC had a reading at the Ford's Theater a month before we opened.
Now, your rep in Boston might render a different decision than what happened for us (after all, each production is different), but we got paid our weekly salary plus a Performance payment. So it would be like, lets say $800 in salary, plus a $282 additional "bonus" for the reading as an example. Basically our reading was incorporated into our regular contract.
But basically, the reading for us counted as a performance, we were compensated, and we went back into normal rehearsals to complete the show for tech week/opening.