How about offering formula in a sippy cup? I'd try this when you're totally out of the house, or maybe start with water first.
I got DS to take one (one of the tommee tippee ones) by first taking the lid off and letting him explore the cup with some water in it, not too much so he couldn't soak himself, also I was keeping hold of the cup so he didn't immediately tip it all out. He could see the water so he was curious and I got him to drink some like this, with me holding onto it too.
He seemed to like this game so I repeated it for a few more mealtimes, holding the cup but letting him control the movement mainly. Then one day I just stuck the lid on it, because it was the same colour as the cup he didn't immediately notice and I kind of guided the cup into his mouth before he could clock that it looked a bit different. The water came out into his mouth as he expected and he accepted it straight away, whereas the first time I'd presented him one (with the lid on) he got upset because he didn't like the water coming out of it - I suppose it was a bit of a shock?
You need to use a free flowing cup for this to work and it has to work well as a cup without the lid - the tommee tippee flip top ones are perfect, although you'll probably have to replace them fairly regularly if using for milk as the spouts are hard to clean, I used to poke around in them with a washing up brush.
If you didn't know, it should definitely be possible to continue breastfeeding in the evenings and even at weekends, your supply can adjust to a pattern like this, so her refusing other milk when you're around needn't be a problem. And if she really really won't take it, you can always instruct nursery to sneak formula into her food - cheese sauces, porridge etc(!) and then once she's 9-10 months she should be fine with food during the day and BF morning and evening.