Setting Custom Properties

I know that I can read/write custom properties of an object by using
the following:

Setting:
document.all['Control'].customProp = "this";
Getting:
document.all['Control'].customProp;

Is there a way I can run code when this custom property is set. Or
perhaps there is a way to create a custom method?

Example:
<span id="MySpan"><input type="text"></span>

I'd the property:
document.all['MySpan'].enabled = true;
To automatically do this:
document.all['MySpan'].controls[0].disabled = !thevalue;

OR the method
document.all['MySpan'].enable();
To do this:
document.all['MySpan'].disabled = false;