At best glide in a C172 (~65kts) with the full 40 degrees of flap you have a stonking descent profile! Approaches that are still high in this configuration should be abandoned and flown again properly. There's never much need to side slip a C172 IMHO.
Considering side slipping however, if the 40 flap with idle power wont bring the bloody thing down (which I fail to believe it wouldn't) and going around for another attempt is impossible it's a forced landing situation. In this case a flapped side slip would be the only option regardless of what the POH recommends. It's probably the only time going against the POH is a good thing if the aeroplane occupants survive.