Where are you based these days? In SJU or up in HYA?
Let scheduling know your need for the hours and I'm pretty sure they will be able to hook you up on whatever night flying is available. Is there still a late EIS turn from SJU? There's plenty of flying in the dark out of HYA - even on ACK Air I logged 3 night hours this week. If you can get late BOS-ACK legs you should get it quick enough.
You probably won't make your 100N by 1200TT, but if you can get an hour a day, it shouldn't be too much of a burden.
As far as buying time, I'd do the math for yourself, but I figure $3500 divided by a $3 raise, times 35 hours a week, equals about 33 weeks before you break even. So if you're getting about an hour of night flying a week, its a wash.
Oh yeah, one more thing, e-mail Bill Cush and find out how many hours of flight training you get for the upgrade ride. They schedule lots of that stuff at night, so you might be able to arrange to start your training when you have X night hours to go, expecting to get the balance while in training.