For example, I am hoping to have a waterfall sound like it is hollow and inside something (hence the hollow). Any plugin suggestions or techniques you can offer?
Actually, how you EQ would depend on the type of environment it's in.
What type of environment are you looking to place this waterfall in?
To accomplish this would definitely be by using EQ feeding into a Reverb. What your settings are depend on the reflective surface characteristics and environment size.
The trick to this is understanding the characteristics of your sound and what different types of processing will do to it. The main frequency component of a waterfall is primarily going to be the high end white noise type hiss and then secondly, it's low end rumble.
Where you choose to cut or boost your EQ will modify the reflective surface characteristics. The reverb will help you define the size of the space it's in.
If you want it to sound "hollow", I'd actually cut out some of the mid - mid-high range. It seems you're looking for a vowel type filter I presume to make it sound like it's in a tube/pipe. If you want to make it sound as if it's filling a pipe/drain or a drinking glass over time, then you would want to automate a slightly boosted swept-mid EQ that goes from low to high.
If the sound takes place in a cardboard box, that surface is less dense, and therefore less reflective, so it's safe to roll off a bit of the highs (I wouldn't do too much though). If it takes place in a bathroom, then you wouldn't want to cut the highs at all as bathrooms have highly reflective surfaces that accentuate the high end.
The reverb is a little more simple and with a decent reverb you can likely find a preset that might only need a little tweaking. You'll basically just adjust it to the size, density and shape of the room you want.
Hope that helps!
+1 to worldizing.
If the waterfall is loud/powerful enough to handle some layering, try adding a subtle layer of empty cardboard or wood box - the bigger the better. Try spraying a hose at it with about the same water speed as the waterfall. Layer it in.