nightcrawler,
I think that may be a viable solution. My concern was with statement such as: "SELECT * FROM ( EXECUTE PROCEDURE xxx()) t". In such case, the query engine needs to know the data types of the stored procedure output to process the query.
If my understand your suggestion correctly, then we will have two types of stored procedures. One type (the existing type) has predefined output parameters, and a new type that has no predefined output. The new type of stored procedure, the ones that has undefined output, will not be allowed in place of table input like the sample statement above.

We need some clarification on this.

Are the output parameters still needed to be defined? I think that the answer is yes. And the select list of the last statement must match the output parameter list of the stored procedure.