It seems you may achieve what you are looking for by abusing the shorthand field using BibLaTeX 2.0 with Biber 1.0 (earlier versions may work as well).
In a long text I wanted to mix authoryear citations of books and papers with several articles from newspapers or magazines identified by an abbreviation of the publication title and a date, eg NYT/20120722 for a New York Times article of that date. Shorthands defeat every other field, like the author field, or title or journaltitle and, as far as I can tell, the are printed regardless of the style.
So if you format every entry accordingly, say with a custom style of your choosing, combining two styles is fairly easy and transparent to the user, even without (potentially ugly) hacks. Other answers, like this one, explain how to have two citation styles in the bibliography.
I think that a more consistent solution would be a third option to \cite
and friends like this: \cite[pre][post][style]{key}
so you would write \cite[authoryear]{Jones2010}
and get Jones 2010
or \cite[25][alphabetic]{Jones2010}
and get [Jon10]
. (I've omitted pre/post fields disambiguation here, but it should be addressed as well). However this may be bring much unneeded complexity and I don't really know if it feasable, let alone desirable.