I think - FWIW - that the problem with the mum reading books not in her language is not so much an OPOL problem as the fact that it raises the question - WHY ISN'T THE DAD DOING IT? Fair enough if he's not home for bedtime - but, especially in the first year, babies are lying down and on the verge of sleeping practically all the time, so any time can be book time! I speak as (A) a father and (B) an OPOL practitioner. I read to my child loads (and will to my second soon; she's only 2 months at the mo!) We take books to restaurants and cafes; if she starts playing up, a story will often calm or distract her. And it's a nice way of spending time together. But then, I'm very bookish myself.
Annie - in terms of turning passive knowledge into active knowledge, after a while I just started telling my daughter I didn't understand when she spoke the "wrong" language (assuming, of course, she is not deeply distressed about something!) Nine times out of ten, she will attempt it in French; I can then "feed it back" to her, ironing out any faults she has made. The other time she will genuinely not know one or more words; I can then find this out and supply the missing words, again by feeding back.
Example - 9 times out of 10;
D: I don't want you to do that!
F: Je ne comprends pas; tu parles anglais.
D: Je veux pas que tu fais ca!
F: Ah, tu veux pas que je FASSES ca? Ah, d'accord, desole! etc etc
Example - 1 time out of 10;
D: I want to wear my fairy wings.
F: Je ne comprends pas; tu parles anglais.
D: Je veux porter mes wings.
F: Quoi, tu veux porter tes ailes ]ideally, showing wings at same time]? Ben, les voila, tu veux que je t'aide?
Obviously, this takes longer than just responding to the initial English utterance and takes patience and an "encouraging" face; but it produces real results.