How to search for a step-up transformer: 10V -> 90V
And, FWIW, yes, I know I could use a step down transformer and just treat the secondary as my primary


Get the datasheet for a stepdown transformer of about 10:1 or 11:1 and look at the difference in output voltage at no-load and full-load. You should be able to use it as a stepup transformer and use the voltage droop characteristics to determine roughly what it should yield under load.

It's fine to use a 120VAC 60Hz transformer at lower voltages (e.g. 90 on a reversed primary), but not at higher voltages -- one of the key limiting factors is saturation; the volt-seconds applied across either primary or secondary (scaled appropriately) can't be increased much w/o bringing the transformer into saturation. (Unless, that is, the transformer is overdesigned, in which case the manufacturer is losing money.)



(update based on comments)

20Hz? You need to includes things like that in your problem statement. If you want to use a step-up transformer, that would require a larger transformer than 60Hz because of the increased volt-seconds. You'd have to use a 60Hz transformer rated at least 270VAC on one side, in order to handle 20Hz power transmission. 

Why don't you search for telephone ringer circuits? I'm not an expert but the usual way to generate odd voltages like this is to create the DC voltage you want, then switch it on/off with transistors, generating a square wave that is then low-pass filtered through passives (e.g. the phone network itself) -- that's how it's done for electroluminescent displays.

Unless the telephone spec says it needs to be, the ring generator doesn't necessarily have to produce sine waves. (this webpage cites several examples of commercial PBX equipment)