I'm planing world domination but that's another story.Depends on what you are planning.
We planned from the very beginning to have some options for heat dissipation / spreading, but we always mentioned we will test how much will be needed when everything is assembled.
I don't think your solution would be much different than one of our options.
In fact, we left enough space between the PCB and the battery compartment to put a full insulation / heat spreading mat inbetween, just in case we need it.
Actualy, I just wanted to find out if there is some small room at the top side of the PCB and case to guide a small, flat heatpipe through the top side of the keyboard area, right between the hinge/lid part. So if there is some room for an metal heat spreader, it would be a nice spot to place such a thingy. Additional air vent holes and your idea of using the SD-card-slots may also be included.
I made some quick sketches of what I had in mind, it's only a Pandora of course so please ignore it:
CPU PCB with heat sink for the SOC:
flat (1mm thick, 5mm wide here) Heatpipe guides heat to the top through the PCB (only small area needed for this) to the heatspreader:
...something like this. ^^"