Wouldn't it be possible to DIY the full project? I.e. you don't need to start with a USB block errupter to get the design, but you could make one out of microcontrollers couldn't you?
I could be way off, but this is something that's sat in the back of my head for a while now, just never really looked into it. I don't see why it wouldn't work.
You could - but it is extremely slow to do it with microprocessors, even the more beefy ARM ones - and at that point you're starting to lean toward building a single board computer anyway.
If you were to do it with an ASIC chip.. well, you're pretty much doing what people have been doing
A DIY-level thing in between would be the FPGA miners. There's a ModMiner Quad up for sale in another thread that's a good platform for starting with, as it's already got the necessary bitstreams.