Richard has a lot of experience in this area, but I will try to summarize. A purely mechanical pump will generally hit full pressure right away and act as an on/off system regardless of pump speed. You would need some sort of variable regulator or bypass apparatus, which just gets more complicated.

Many have tried, unless much of the support stuff is in place it's more money and hassle then common options.

Post up what you have in mind, we love to see new stuff and ideas.