The obligation to fast starts on Yom Kippur itself, not before. Once Yom Kippur arrives, it's forbidden to hook up to intravenous, since blood will spill.
And before Yom Kippur, there's no obligation, per se, to prepare for the fast. Therefore, there's no obligation to hook up to intravenous.
And since there's no obligation to "eat" intravenously, it might actually be forbidden to do so if you don't need to. For one, inserting a needle is a transgression of the prohibition against unnecessarily wounding oneself. And who knows, intravenous may involve certain health risks, all of which may not be known at present.
- Iggrot Moshe Orach Chaim 3:90