Whenever—
(1) the United States, or any of the Territories, Commonwealths, or possessions, is invaded or is in danger of invasion by a foreign nation;
snip
the President may call into Federal service members and units of the National Guard of any State in such numbers as he considers necessary to repel the invasion
Let's get some rational thought engaged, okay?
1) Slavery is not permitted in the US. Captured illegals cannot be forced to build a fence as part of any sentencing -- regardless of the zero probability of this sort of thing surviving a "cruel and unusual punishment" challenge.
2) The US prison population of the US is about 3 million people. And they are full. Many are full with illegals who committed other crimes. Given that they are full, there is nowhere to put illegals who are awaiting a deportation hearing. Don't suggest tent installations. Even if they were funded, there is no way whatsoever that would survive a concentration camp array of verbage. It's Not Going To Happen.
3) From day one, improved border security has been step #1 of Bush's plan. All attention was focused on later parts of that plan, but border security was always the first item. Bush can move rightward on this matter without changing anything significant in his plan by simply inserting a time line. Seal the border. Then Wait 6 Years. There will be many fewer left at that point. Only then look into dealing with whoever is left after the attempts to visit family at home result essentially in self deportation (because they can't get back in).
4) Because US demographics define a situation where the reality is NOT one of "work that Americans refuse to do", but rather "there are not enough working age Americans to do all the work a 12 trillion dollar economy has", only after 6 years of sealed border do we address punishing those still present and shipping them to the border for recycling.