Cesar,
Their is no way to do this based on load today. One question I would have would be what are you using your dedicated AMs for today? The Aruba system can do many AM functions simultaneously while also serving as an AP, including spectrum, WIPS, packet capture, etc. Trade-off typically would be rogue/adhoc/client protection effectiveness (i.e., DoS'ing or Tarpitting rogue AP). Is it an option to place the AM into AP mode in your environment without major trade-off? I assume so since you would like to see this done dynamically.
Regards,
Austin