If you are using a laptop, you already have a more sophisticated version of such a thing: it's called a heat pipe. The heat source is your processor chip, and the heat sink is a distance away, next to the vent van. The fluid is usually methanol rather than water.
Yes, this can be constructed, but not without care. Obviously you need to calculate how high the steam pressure could go under the worst case heat removal conditions given your volume, area and how much water you charge the system with initially. Since you have no relief valve, you will be relying on overtemperature detection on the heater and/or intrinsic design to protect against rupture due to overpressure. If your steam pocket chamber is beyond 1.5 cubic feet and has a dimension beyond 6", it's an ASME vessel which requires a relief valve. And if your external environment is subject to corrosion or cracking mechanisms, the risk of a leak or catastrophic failure will always be there. With 6 kW of input energy, the amount of energy stored in the vapour/condensate will likely not be trivial.
If steam is available, it's preferable to use it, provided that you have a means to remove the condensate at pressures below atmospheric, i.e. presuming your condensing pocket surface will be below 100 C.
