Ok, it's been covered that the fog lights can be used in place of the low beam headlamps under certain environmental conditions.

However, during the day you are not required to have your low-beam headlights on, just some form of DRLs.

So having just the fog lights on as DRLs, you're not technically using them in place of your low-beam headlamps. You're using them as DRLs.

You're fine to have your fog lights act as the daytime running lights.

But if I'm not mistaken, the fog lights MUST turn off when your high beams are activated, regardless of what you are using them for, as you can only have 4 forward facing lights turned on at one time.