If you have a 100W electrical load and you drive 100W plus efficiency losses, say 110W, into the generator, things will be in a state of equilibrium, with 100W being converted from mechanical input power into electricity, and the other 10W of mechanical input power being eaten up by losses.
Now suddenly put 1kW of mechanical power into the machine; at that instant, before the rotational speed can change, the 100W electrical load will continue to present the same mechanical load to the prime mover. Things will not be in equilibrium, and the machine's rotational speed will accelerate. Depending on circumstances, this may or may not increase the electrical load. Certainly the generated voltage will go up, and any simple resistive load will therefor absorb more power, but maybe you have some regulation such that the load continues to draw exactly 100W.
So assume the load continues to draw exactly 100W. Where does the extra 900W of mechanical power go then? The machine's speed must increase until the losses equal the mechanical driving power; so it ends up turning extremely fast, the increased power going into increases in friction in the bearings, windage loss due to the rotating parts, eddy currents in the magnetics (and doubtless a couple of other things I forget at the moment), none of which are desirable.
You would find that, without exceeding the machine's electrical rating, you would quickly exceed its mechanical ratings, i.e., probably long before you got to 1000W, the rotation speed would be several times the suggested speed, and catastrophic failure would likely result. Note you can do this with no electrical load on the generator at all.
For data transmission / reception, one of the less expensive options today is a pre-built module around the nRF24L01+ Transceiver IC. These modules typically offer a built-in PCB-trace antenna, 250 Kbps to 2 MBPS bandwidth before error correction, and are tried and tested.
Most important, they save you time in debugging and antenna tuning. After thousands of people have used these modules, which are built on the manufacturer's reference designs after all, most of the kinks are pretty thoroughly ironed out. Also, being able to tap the experience of many others on the internet who have used such a module, counts for a lot when trying to resolve issues.
For instance, this listing on eBay is for a mere US$2.10 with free international shipping. It uses the 2.4 GHz band, which does not need licensing for low power use in most countries.
Another alternative is this 433 MHz band transmit / receive pair of modules (just 9.6 Kbps though), in case you specifically want to stay with transmit-only and receive-only designs. US$1.99 for the pair makes it pretty attractive.
Of course, in each case, you could as well build your own module starting from the IC manufacturer's reference design, and thus learn while implementing your radio functionality.
It is unlikely that the price advantage of massive volume production can be beaten, though.
Best Answer
Back when I was in grade school I had a crystal radio set. A crystal radio contains no amplifier. The output signal is completely powered by what is picked up from the antenna. I had around a 50 foot length of wire running out my bedroom window to a shed in the back yard as a antenna. With that I could pick up a 50 kW AM station over 20 miles away quite clearly. It was reasonably loud with headphones, a few kΩ impedance. I hooked up a impedance matching transformer to drive a 8 Ω speaker with it. The radio program was easily audible with my ear up to the speaker. Sometimes I left it on at night to annoy my brother. You couldn't make out what was said accross the room, but you could hear what sounded like distant talking, enough to be annoying if you didn't know what it was.
I can't say how much power that actually was, but enough to harvest and use by a low power intermittent device is possible.
As for loading the transmitter, that only happens in the near field. The radio station I mentioned broadcasts at 1.03 MHz, so the wavelength is about 290 m. For anything much beyond that, the power is already propagating irrevocably from the transmitter such that it can't see any loading. Put another way, the transmitter has already been loaded with that power, whether you use it or let it propagate into space forever. Since I was over 20 miles away, I was well past the near field. My receiving the signal only reduced the field very slightly in the vicinity of my antenna. As far as I know, there is nothing illegal about using a 50 foot wire antenna completely on your own property to receive AM radio stations.