OK, let's cut to the chase. I think we're talking about a specific diconnect: the Square D QO200TR http://www.drillspot.com/products/44409
/Square_D_QO200TR_Air_Conditioner_Disconnect_Switch

This is a nifty little disconnect - you can swap out the 60-amp switch with the QO breaker of your choice. Now one is tempted to ask: what if I tap off one of the legs on the 'in' side, and power my receptacle with that? (Anyone tempted to 'double-lug' here?)

Here are the problems with using the disconnect switch that way: you don't really have room for those splices, there's no place to land your neutral wires, there are not enough ground lugs, and there's a real issue in finding a place to land that third connector.

As an alternative, you could use something like this panel:
http://www.grainger.com/product/SQUARE-D-Load-Center-5B756?s_pp=false

Combine that with a breaker like this one and you're home free: http://www.superbreakers.net/homt20...;utm_medium=ppc&utm_campaign=product

As a caution: Ignore the prices, as they bear little resemblance to what you'll find at the local supply house.

Otherwise, let's imagine a 12/4 cable (2 hots, neutral, and ground) feeding a junction box. From that box it continues direct to a duplex receptacle. Also, from that junction box, you splice off to feed an HVAC disconnect as well. Is this allowed?

Well, let's look at the tap rules. The tap rules all assume that there is overcurrent protection at the tap. So, if there is a breaker or fuse in the disconnect, the disconnect is the 'tap' and you are legal. The problem is that you're now limited to the rating of the receptacle; most HVAC will want more power than 20-amps.

I'm not aware of an easy way to provide the overcurrent protection at the receptacle- unless you want to use another fused HVAC disconnect there. That could work: a gutter for the taps, feeding a row of disconnects.

Greg, I think you're over-thinking the FLA issue. That rule would apply only to the rating of the final overcurrent device- that is, the fuse / breaker in the disconnect. It would not apply to the ampacity of the feeder.