I started a thread searching for base rock in UK and I managed to come across Ocean Rock (the ONLY stuff I can find anywhere, eBay etc..)
I actually have a piece of it, weighs about 6kg and would make some nice base rock broken up.
Here is a pic:
It's quite porous although I'm not too fussed about it eventually becoming full live rock, as long as it takes on the colours and front of live rock, it will be fine.
The idea is, if I can use this rock, ideally it would seed like normal base rock will but if not, if it at least has coralline algae growth and such, it will serve its purpose fine.
Just to bulk out my current live rock really.