Backflow valves at every branch in the water mains are going to be needed to localize any problem. Something else to worry about.
In the end, when this and other anti-terror measures are installed, we will have a safer nation---but the cost will be high.
This is the only thing that would work. If I were a terrorist and there was a backflow preventer on the plumbing where I wanted to inject the poison, all I would need to do is disable it.
BTW, a bicycle pump or vacuum cleaner would have a lot of difficulty overcoming city water pressure. Not sure I understand how that would work.
What would work beautifully is the wide variety of pumps available for various purposes. Just look in your Gringer's catalog. Literally hundreds of workable pumps can be purchased from under a hundred dollars and up.
I used to do pressure washing, so I have some familiarity with pumps. Trust me, this ain't rocket science. I could do it in a couple of hours if I wanted to, starting from scratch.