I would think 24VAC may be able to power 12VDC LEDs directly: because LEDs are diodes, they will block half of the AC waves. So a 24VAC RMS voltage becomes an effective 12V after passing through the LEDs. However, I am not sure about the reverse breakdown voltage of these LEDs — it might be quite low so they can’t withstand the large negative voltage incurred by 24VAC. Another possibility is to have two of these LED strips connected in series, so the 24VAC voltage gets spread on two 12VDC LED strips and that should much better address the breakdown voltage issue.