Your most crucial problem is that 100mV will not turn on
any "regular VT" device, and any "native" (VT ~ 0) device
will not turn off full (so very wasteful of input power).
To run a decent boost backend you would need to ring
up a supply nearly as large as what you hope to create,
just for gate drive. That alone will probably consume
all or most of your microwatts.
You should look at what people are doing & have done
in RFID tags, which have similar (or maybe identical)
challenges.