How to use the DC in a DC offset AC signal?
No, it is completely impractical.
First, your computer can't just 'filter out' a 120VAC waveform on top of the DC.  You could actually do that, if you had a rather large choke, but that would be large and heavy and have to be connected between every outlet and 'DC Device' that you want to power.

For similar reasons, you would also need to have a large capacitive isolator to filter the DC out from any AC device you might want to power.  AC devices don't typically do well with a DC current on them, which you would have unless you blocked it.  To DC, a transformer or motor coil looks like a dead short.

Unless you completely filter out the unwanted component, your plugged in devices will fail, and sometimes spectacularly.