I haven’t tested this across any browser platforms and Flash versions, but if this works for you, then it would rely on undocumented behaviour that we won’t support.
beginFill takes a 24-bit colour (uint) and an optional alpha (defaults to 1.0; opaque). There’s no mention that the colour can also include an 8-bit alpha value in the unused byte.
Looking at the edge cases, such an interpretation would not make sense. For example, in the absence of the alpha parameter, a colour value of #00FFFFFF would have be interpreted as having alpha = 0, rather than default to 1.0. Similarly, a colour value of -1 is bitwise equivalent to #FFFFFFFF and would be interpreted as having alpha = 1.0 (opaque; not transparent).