Envelop detection with low sample rate?
Is the ADC sample-and-hold fast enough to even capture any envelope peaks or even half cycles?  If not, all bets are off.

If your ADC does have a fast enough capture time, then randomizing your sample times might be a good bet at capturing a few near extrema.  If you know, a priori, the approximate shape of your waveform, then Monte-Carlo sampling might allow one to estimate the waveform amplitude from the statistical distribution of enough sufficiently time-randomized samples.  Assuming the envelope changes slowly enough.