Semipermiable rock mass and drain tile?

I was thinking about an outdoor RMH for an outdoor patio space, and was wondering if anyone has tried using gas permiable rock as the mass and part of the exhaust channel. I suspect that this might not be possible due to restriction, but hear me out.

Imagine a regular RMH outdoors. A 30' long bench, either straight or with one 90 degree turn, and a vent out the far end of the bench. So far, pretty basic. Except that this bench is a box like structure holding loosely fitted gas permiable rock, such as common gravel, and that the vent channel has numerous holes along it's length smaller than the average size of the rock pieces. Once the RMH is fired up and the fire channel and riser are permitted to come up to full temp, the end of the exhaust vent pipe is partially or completely covered in order to deliberately force hot gasses into the permiable rock. Assuming it's actually outside, the co2 and co shouldn't be a risk for anyone, but the process should allow the mass to heat up even faster than a normal RMH would, since a good portion of the hot gasses would come into direct contact with more of the mass. Another method would be to seal the exterior of the rocks mass, and have a second, parrallel vent tube with more holes in it that would permit the hot gasses to travel from the first vent tube, through the rock, and towards the second vent tube before traveling further. Sealing the exterior might permit it to be safely used indoors this way, but I wouldn't want to be the first guy to attempt it, at least not with my family in the house and not without a professional grade oxygen and CO monitor.

I wonder how I could put a useful surface on the top of loose packed gravel that would be weather proof, any ideas?

Well, have you thought about friction? I think 30 would be too long.

Check this one

http://donkey32.proboards.com/index.cgi?board=experiment&action=display&thread=560

And this one

http://donkey32.proboards.com/index.cgi?action=display&board=pictures&thread=609&page=1

I would use either thic or a brick, concrete, stone or something similar bell. That's direct contact too, and less friction than the permeable rocks in flue you're sugesting.

HTH.

The end of the bench closest to the stove would get way too hot too fast.

You could do this, but don't perforate the pipe until 1/3 of the distance from the end. Put the holes in the bottom of the pipe.

Suggestion: Make a test one using chainsawed pallets for the bench sides, 2' concrete patio stones for the top, and a pile of rock that you can use on your driveway. That way, your worst case scenario is having to replace a bunch of holey pipe.