It's funny you should ask this as I delve into sound effect programming... or maybe opportune timing
The answer is "depends". For field, the channel volume can be set and so you can do what you like.
For battle, as far as I can see, a lot of it may be hard coded to just use volume 127 no matter what. It's very messy in there. A sure way of making it work would be editing the wav file itself for Ultima, as you note.
It still could be that there is a table in the exe detailing the volume to be used effect by effect for battle, but I wouldn't get your hopes up. It's far more likely, since most of those effects are decided by the same function, that they are all set to the maximum by default. FF7 doesn't have a "play effect at volume" operation that I can see. It sets channel volume - and I don't think any channel volume is being set regardless in battle.
I mean, a really bonkers way would be to force a channel volume change in asm and then change back after Ultima. But that is just so much nonsense. All in all... editing the wav is your best bet. When/if I get round to releasing my battle effect replacer dll, this would be even more of a doddle.