Using a HRV/ERV Fan without mounting it on a wall/window?
The question is rather broad as asking mostly "is this feasible", and therefore maybe off topic, but it's an interesting question and here's what comes to mind...

I picture a panel, either hinged to the interior of the wall or to be mounted on shelf brackets or similar and with some sort of clamp to press it against the window casing. This panel would be a sandwich of plywood or other sheathing panel and extruded foam sheet. Your ERV unit would be mounted to it appropriately, and foam tape around the edge would provide an airtight seal. 

Now, one has to consider the real-world ecological payoff of such a thing. Every component, down to the screws and adhesives, has an ecological cost associated with its manufacture, shipping, and implementation. Is that really worth doing? What's the payoff time when compared to the pollution and energy savings offered by the ERV? Are there other low-hanging fruits that would make more sense, such as reducing hot water consumption and sealing leaks in the building envelope? 

I admit that I'm not the type to have an interest in detailed investigation of that question, but it's worth considering at a high level. Good luck.