I hadn't thought about only working on your own blockchain like that.
A piece I'm missing is the heuristic most clients use to determine "longest". Depending on how that works, this plan could be really easy or really hard.
I believe the way longest chain is determined is the sum of the difficulty of all the blocks. This basically boils down to just the chain with the most cpu power that went into it. So as long as you control the majority of the cpu power, you can control the longest chain.
A piece I'm missing is the heuristic most clients use to determine "longest". Depending on how that works, this plan could be really easy or really hard.