Low-threshold MOS transistors might be usable
but you will be in a bind between back-leakage and
forward conduction with maybe 2-3 decades of
current between "on" and "off". That doesn't bode
well for energy-capture efficiency.
You could look to 3.3V technologies (2.5V being
a bit short for the desired voltage) which offer
multiple VTs, and go play. 3.3V is also a common
"I/O device" in dual gate ox technologies and may
(or not) have a low threshold, thick gate combo
in the device set.
Now if you want a breadboard, built from discrete
components, that's unlikely.