Thanks @fge for putting me on the right track. Jackson has @JsonAnySetter and @JsonAnyGetter annotations that can be used to do this:
public Foo {
public int bar;
private transient Map<String, Object> params = new HashMap<String, Object>();
@JsonAnySetter
public void set(String k, Object v) { params.put(k, v); }
@JsonAnyGetter
public Map getParams() { return params; }
}