I did exactly this in a little grow box I ran last year. The diffuser (with just water, no oil) worked well while it was a seedling but once the plant got a bit bigger it wasn't really effective enough. I found a dish of water with the fan blowing on it was much more effective then.
But sure - while they're babies it works well. I had it sitting low and it kept the canopy area just nice (around 55% RH I think)
Hope you're enjoying the forums. It's a great place to hangout and learn and share about growing and using Cannabis for many different purposes Not to mention all the delicious eye candy on offer !
Have fun getting around the rooms. If you need help, just holler! Most everyone here is super helpful and very nice.