What's the spacing between the dropouts? I think the main physical limitation here is whether the hub is going to be wide enough, given that you've got already got the euro bb sorted.

One thing to keep in mind is that 25/9 gearing is going to be a lot higher on 700c wheels than it is on a 20". We're talking like 74 gear inches vs 51 or so, which is a bit of a leap. There's going to be a lot more stress on the teeth than there was with the same gearing on the bmx (you might very well end up trashing the driver and/or sprocket the first time you need to haul up a hill). It'll also be hell on your legs going from 51 to 74.

Assuming the hub will fit in the dropouts, my suggestion would be to scrounge up a 22t sprocket and use that, if you're set on the 9t driver. 22/9 on 700c gives you about 65 gear inches, which is probably a better starting point coming from bmx. It's high enough that you won't be spinning out constantly (especially since you'd be used to coasting downhill anyway) but low enough that you'll be able to get rolling fairly easily and climb the odd hill or two.