Beta carotene (prOformed) can not cause an overdose...it is converted to vitamin A as needed....however, its controversial as to whether any/all chameleons can convert it or not. As you said...PrEformed vitamin A can build up in the system.
Its fairly hard to overdose the calcium...although if you really went overboard with it and the D3 from supplements it might be possible. D3 from supplements can build up in the system and lead to health issues. D3 produced from exposure to UVB shouldn't lead to an overdose as long as the chameleon can move in and out of the UVB.
In addition to this, vitamin A, vitamin D3 and calcium along with phosphorus need to be in balance for good bone health and for other systems in the chameleon. Excess prEformed vitamin A may prevent the D3 from doing its job and lead to MBD.
Exposure to proper UVB, appropriate temperatures, supplements, a supply of well-fed/gutloaded insects, water and an appropriate cage set-up are all important for the well-being of your chameleon.
Appropriate cage temperatures aid in digestion and thus play a part indirectly in nutrient absorption.
Some of this has been said already...but I hope it will help...
Since many of the feeder insects have a poor ratio of calcium to phosphorus in them, its important to dust the insects before you feed them to the chameleon with a phos.-free calcium powder to help make up for it. (I use Rep-cal phosphorus-free calcium).
If you dust twice a month with a phos.-free calcium/D3 powder it will ensure that your chameleon gets some D3 without overdoing it. It leaves the chameleon to produce the rest of what it needs through its exposure to the UVB light. D3 from supplements can build up in the system but D3 produced from exposure to UVB shouldn't as long as the chameleon can move in and out of it. (I use Rep-cal phos.-free calcium/D3).
Dusting twice a month with a vitamin powder that contains a beta carotene (prOformed) source of vitamin A will ensure that the chameleon gets some vitamins without the danger of overdosing the vitamin A. PrEformed sources of vitamin A can build up in the system and may prevent the D3 from doing its job and push the chameleon towards MBD. However, there is controversy as to whether all/any chameleons can convert the beta carotene and so some people give some prEformed vitamin A once in a while. (I use herptivite.)
Gutloading/feeding the insects well helps to provide what the chameleon needs. I gutload crickets, roaches, locusts, superworms, etc. with an assortment of greens (dandelions, kale, collards, endive, escarole, mustard greens, etc.) and veggies (carrots, squash, sweet potato, sweet red pepper, zucchini, etc.)
As I said before, Calcium, phos., D3 and vitamin A are important players in bone health and other systems in the chameleon (muscles, etc.) and they need to be in balance. When trying to balance them, you need to look at the supplements, what you feed the insects and what you feed the chameleon.
Here are some good sites for you to read...
http://chameleonnews.com/07FebWheelock.html
http://web.archive.org/web/200605020...Vitamin.A.html
http://web.archive.org/web/200604210...d.Calcium.html
http://www.uvguide.co.uk/
http://web.archive.org/web/200601140...ww.adcham.com/