Guessing that there are two main scenarios there:
1. A leaky house where positive-pressure causes flows to come out of various leaks all around.
2. A well-sealed house where positive-pressure causes flows to leave through a well-defined channel.
The first-case might be harder to recover heat/coolness from, as the (high/low)-temperature air might leak in a way that'd be harder to make use of it.
The second-case would seem to offer a stream of warmer/cooler air that could be used with a heat-exchanger or heat-pump or something.
To note it, well-sealed houses might have issues with relatively poor ventilation, making positive-pressure more desirable -- much like how the OP describes wanting to keep CO2-levels down with their positive-pressure system.