It's a fun thought Jesse,
I'd have trouble imagining semps taking over an entire landscape, but who knows I suppose it's possible. In my experience as a hiker, wild succulents tend to be among the most timid of plants, thriving in small patches but not found anywhere else for miles, I'd expect sempervivum would behave in a similar fashion. Sure they grow quickly in a garden where they are nurtured, but without weeding or watering I imagine they'd become quickly overgrown and starved for light if they tried to spread to the ground beneath those nice rock crevices. Native weeds are so much more aggressive, at least here in Oregon. Generally I lean towards the environmental protection standpoint, but I can see how one might be tempted.
On a less controversial note, I've often thought about planting them in downtown Portland, sneaking them in between buildings and tucking them into cracks in the concrete like hiding easter eggs around. My kind of mischief!
-Sol
