We have been trying something similar too but with no success. Trying to get the asset contents passed to a metadata field on the asset so that we can use that field for RSS description.
%asset_contents^maxwords:15%
Looking like it's a bug just on that one keyword but we haven't found a work around to get the contents some other way.
Seem to be quite a few places where keyword modifiers work in one place but not in others - same kind of issue occurs with asset listings and upcoming event listings: one supports user specified paint layout assignment, and the other doesn't.
We've done the meta description in design by having a standard page asset included via a nest_content customisation, which seems quite inefficient, but at least it works.