I believe the purpose of the sealer is not just to act as a barrier, but to infiltrate the grout and protect against mold and such that can get into it and grow. If you did create a barrier as you described, you'd still need a proper sealer. As to the effectiveness of using epoxy I'm doubtful, but it might depend on where used. For example, a kitchen counter would be much different than a floor that's heavily travelled or a shower.