from what I can tell, fromundaman isn't completely crazy...at least with nair. If you do a SH nair, you can't buffer into an fsmash, but if you do a SH nair and fast fall as soon as you're able (apex of your jump), you can buffer an fsmash pretty easily. There's still lag, but with the knockback of nair, you should be clear to land the fsmash before your opponent can get to you. I've done very little testing so far, but I'll keep you all posted on what else I figure out.
edit: more info- well, we really need a video to explain this well, but the way in which you know you're fast falling with the correct timing is as follows. As we know the nair has two kicks. We can use the sounds of the specific movements in the air to explain the correct timing.
The timing of a regular SHed nair is this: jump sound, kick 1 sound, kick two sound, landing sound. that's four sound effects total.
The timing of a "correctly" SHFFed nair is this: jump sound, kick one sound, then some hybrid sound of the second kick and the landing sound TOGETHER.
If you SHFF the nair correctly, you can buffer your next move almost immediately after you hit the ground. I've been experimenting with viable followups, and I really like a stutter stepped fsmash. If you connect with either kick of the nair at low to medium percentages (<50% or 60%?) the stutter step will put you within range of your knock back to perhaps land the fsmash.
Another way to tell you're doing this "correctly" is that you'll be able to buffer SHFFed nair into SHFFed nair over and over again. This may also be a viable approach against shorter characters, but it's hard to say.
It's up in the air for me if this has any real competitive usage in real time, but I'm exceedingly optimistic. As for the SHFFed fair, I'm significantly less optimistic. The lag on the SHFFed nair is minimal (relatively speaking) but the SHFFed fair, as far as I can tell, still yields pretty ugly lag. Not to mention, it'd be hard to follow up a fair with anything because if you connect with it, then dat mutha f**** be flyin'!
Side note. I'm also experimenting with SHFFed bair again, and I think I might be getting somewhere. For anyone else testing nair and fair, try bair also, and see what you can get.
More to come later. I'm glad to have something new to experiment with.