[Added five minutes later:]
Perhaps unsurprisingly, googling 'flashbacks playwriting' does produce some relevant stuff. (And yet I'm surprised -- I guess I haven't completely grokked yet the now-eternal verity that Google Knows All.) Various useful info found by following the links on that site, including the cool page with examples of hyper time, which I don't think is actually relevant to what I'm doing in this play, but would be much fun to fool around with in another.