What would be best is to contact a couple of stucco companies and ask if going over the shingles will void the warranty. BUT, even if it doesn't, the aforementioned reasons of 'moving shingles' and such should be enough of an alarm to NOT do it. Also, there's always talk about Seller's Disclosure. If you're not familiar w/it, ask any Realtor for a form. In some states, it's a voluntary disclosure. In others, like in DE, it's mandatory. The think here, is, that you may have to DISCLOSE to a potential buyer that there's asbestos on the property. If they find out it's UNDER their siding, the next thing you know, they'll think they'll be breathing it and eventually die from it if they buy your house. I'm sure you know the kinds of thoughts that the mentioning of the word 'asbestos' conjures up. By covering up the shingles, you may have to disclose. And if you DON'T disclose when asked, or lie about it, you could be setting yourself up for a lawsuit.
In My Opinion, have it removed to the sheathing and properly prepared to 'handle' a stucco application. My best to ya and hope this helps.