on the other side it could be very healthy to the renewables industry if all miners would be forced to use only green energy, since this would generate a significant amount of money flowing into those industries. But I don't see any technical way to really enforce that.
I don't see the need to enforce this technically. People acquire electricity through the existing economy. If it is wanted to have a larger fraction of green energy, a way to do so might be to tax energy from non-renewable sources.
I don't think such an obligation is going to work well in practice because it will put strain on renewable energy makers (because they are not big as coal/oil/gas companies) to make enough to meet the miners' demands, and this could cause a global renewable energy shortage as a result. That's going to stir up environmentalists even more against bitcoin, except this time they'll be able to squarely blame bitcoin because of whatever merged BIP mandated this switch.