Please check out non-blocking client library – is it what you are looking for?
(You mentioned that you are aware of the async client features, but I'm not sure what exactly you had in mind)

No, as I wrote: "I'm aware of the async client features, but this is something different."
Consider a PHP script. I'd like to fire (and forget) a query and continue with the script without waiting. I don't (and can't, in case of PHP) want to manually babysit MySQL connections.

This is basically INSERT DELAYED, isn't it? But also for DELETE and UPDATE. And with cleaner internal implementation (currently there are engines that do not support INSERT DELAYED — which means to me that it doesn't look like a normal connection thread, and it needs to be fixed).

It's INSERT DELAYED on steroids, but it shouldn't be limited to insert, delete or update, it should be even faster as you don't have to wait for the query being send and it allows for an initial but very useful client-side-only implementation.

I have a suggestion. Instead of implementing this in the client lib, it could be implemented in the server. So stored procedures could do something like:

EXECUTE IN BACKGROUND <some_sql>;

I hope this is possible.

f_razzoli, yes, this should be easy. It's pretty much the same as an EVENT with ON COMPLETION NOT PRESERVE.

Thanks. CREATE EVENT could be an alternative too, except that it doesn't work inside stored procedures