Here's a pretty damn dumb question.
Can 5v regulators just be stacked up to get
more than 100mA?
It's not generally a good idea to run regulators in parallel to increase
the current rating, unless they're designed for this. If they are, then
the data sheet will indicate this (and will probably give an example
circuit).
However, (a) 3-terminal 5V 5A regualtors exist and (b) the data sheet of
many 3 terminal regulators shows how to increase the current capability
using external transistors.
That siad, remember that linear regulators are very inefficient (power
equal to the load current mulitiplied by the different between the input
and output voltages goes out as heat), and that you might be better
considering a swtiching regulator for such currents.
-tony