It's nice to clean hub bearings up occasionally, but rarely necessary. Any decent hub is sealed well enough that it won't really need fresh grease over a typical lifetime unless it's submerged. Even then, I think only the freehub's thinner lubrication would be easily washed out. Besides, if you filled the hollow center of a fat modern hub you'd probably more than double the weight of the hub, and add some drag between the grease and the axle. Not sure where else the grease would go. It would fill that before trying to force itself out of the seals. I also don't know if the seals would like that. You'd need an old hub with simple single seals and a very small hub body diameter for this to be practical.
It makes more sense on pedals, where there is often only a cartridge bearing on the outboard end and just a large greased bushing surface to handle the inboard load, with minimal seals and minimal empty space. It's probably that bushing surface that benefits from the grease.
Do you have 13mm, 15mm, and 17mm cone wrenches? Repacking hubs isn't difficult, and there is some pleasure involved in putting a clean hub back together. The only tricky part can be lubing the seals in the right spots, but if you just completely coat those in grease during installation they'll be well-lubed. You could easily repack two hubs and lube up the freehub in under an hour once you're familiar with the process.