Can I use thermal paste instead of solder for a SMD heat sink?
Adding some actual number work to your question:

If the Pad is hard to reach, I am inferring you mean it's a slug-pad under the LED, hidden entirely in the plastic casing, such as on the Avago AJSD 1W/3W types.

These slugs are very small, in the case of the mentioned type they are about 2mm in diameter from memory, which is Pi mm2 in area (A = pi*r2, with r = d/2 = 1).

Assumption:

For the next bit I am going to assume it's a 3W LED, because it is unfair to use the slug size of a 3W LED and then sneakily input 5W into the rest of the equations. I'm also going to assume that 1W of electrical energy is converted into light, is it? Depends on the quality of your LED, but... eh.... That leaves 2W of heat to be transferred.

Numbers game:

If your solder paste has 2W/mK, this means that a ten micron thick layer of paste will get you a thermal increase of 3.1K across the barrier: That is probably fine in most occasions. But then you have to guarantee 10micron layer thickness.

So with the same thermal paste, if you use 50um thick, you get 16K of thermal increase between the slug and the PCB. I expect with a little pressure you can guarantee 50micron layers of paste when you are careful soldering the pins, but I'm still not 100% sure without knowing the quality of your PCB and type of LEDs. But don't push too hard, because then the LED will "relax" after mounting and suck little peaks and valleys into the paste, causing reduced contact area.

But consider if you could live with 16 degrees C of temperature increase. I don't think I'd like the challenge of keeping the PCB cold enough to guarantee a junction of below 120degrees C. Not to mention that you should prefer keeping the junction at or below 80 degrees Celcius for a decent lifetime of the LED.

Assuming the Junction to slug is about 5K/W (a number I'm just plucking from the air as an estimate, refer to your datasheet), that's another 10K lost with the 2W in heat generated. We're already at 26 degrees heat up, without considering the transfer properties of your PCB. Will your immediate environment be at or below 25 degrees, always? Then your task is till quite easy. Can it be 50 degrees sometimes? Well, you're sure to not achieve 80degrees Celcius for your junction in that case.

Compare the thermal conductivity of reasonable quality thermal paste with solder:


Tin/Lead - about 50W/mK
Tin/Silver - about 78W/mK
Other Lead Free types - between 55 and 70W/mK generally


That means that Tin/Lead solder can conduct 25 times as much heat away at the same temperature difference as the previously mentioned Thermal Paste. Which means that the slug to PCB in the case of the 50micron distance would only give a temperature rise of 0.6K. That pretty much means that this specific example LED would fare reasonably well with even 50 degrees Celcius environment, where the thermal paste one might not always.

Will it work for testing? Most likely yes, it will transfer quite a lot of the heat away and you'll probably notice it getting a bit hotter, causing the colour of the light to change slightly, it may change a bit in brightness too, but they won't instantly die on you.

Is it a good idea for production? Probably not. You should always account for worst case when building a device for production and your worst case isn't a nice smooth full-contact thin layer of paste under a LED, as I assumed to save myself some annoying maths.

Put simply, your worst case with solder is more than an order of magnitude less bad than the one with thermal paste. Not to mention, if you get the soldering right, with the right temperatures and flux levels, the chances of bad thermal contact are significantly lower as well. (It expels air-bubbles and moisture when it's hot and it likes to flow out across all the metal surfaces)

Quick Maths section:

The formula for thermal conductivity is explained here (why type it again if I don't have to): http://www.engineeringtoolbox.com/conductive-heat-transfer-d_428.html

Which you can rewrite as dT = (qs / kA) - units same as on the linked page.

The assumption of 2W/mK for thermal paste comes from 0.5W/mK for non-brand leaky types to 3W/mK for normal electronics high-end or early graphite types. There are pastes with beyond 10W/mK, but they are messy and sometimes have other corrosive properties that you have to be very careful of. My most expensive, safe in all uses paste is a graphite-silicone composite at 4W/mK. It could be that the Gellid Ultra's are safe enough though, I believe they go at about 7 or 8W/mK? But still, much less than solder.

Data about thermal conductivity of solders came from: 
http://www.electronics-cooling.com/2006/08/thermal-conductivity-of-solders/

That should allow you to do your own feasibility study with the sizes and parameters of your own LED.