That would be great! We currently have a use case where we need to talk to more than one client, so that would make things cleaner.

The following code line from current version (2.1.9) and both Query and Mutation gives precedence to props client. So we can indeed override the default client via props.

// https://github.com/apollographql/react-apollo/blob/master/src/Query.tsx#L167
// https://github.com/apollographql/react-apollo/blob/master/src/Mutation.tsx#L120
...
this.client = props.client || context.client;
...

You would use like:

<Query query={query} client={client}>
...
</Query>

The feature is not documented though.
In addition to that the most common approach would be having a Gateway API running GraphQL and managing queries to different targets APIs. I wrote this article discussing some approaches if you really want to do that and pointing out some pros and cons.

Please PR any docs you would like to see. Client prop is definitely not going away.

Documented here: https://www.apollographql.com/docs/react/api/react-apollo.html#query-render-prop