Jake,
there is no way to do this. The 100mA rating is not put in there
intentionally, it is just physics. Any transformer and related
circuitry (like a voltage stabilizer) will generate heat when in
use. At some point the heat becomes too much for the circuit and
the packaging and you run the risk of failure and possibly fire.
Play it safe, get a transformer that can handle the load.
Remember that these little power supplies are _not_ current
sources so you don't need to be afraid that a 3V/10A rated
power supply will distroy your Omnigo. What is important though
is that the voltage level is very stable. The advantage of
batteries is that they put no noise in the circuit. I therefore
recommend that you get a powersupply of good quality in a
reasonable power range (700 mA or so).
best regards,
Sander