This isn't recommended since many browsers and proxies do not cache resources that are referenced using a query string, even if a cache-control or expires header is set appropriately. Google says Squid up to 3.0 will not do so:
Fair point and thanks for the link -- I've only seen this phrased as a warning against "some proxies" that I've never personally encountered. But I can live with it. If your proxy is broken in this way, you will have to fetch the asset from the CDN rather than benefit from your local proxy.
To be fair, I think it was still the current version up through 2011.
The more important point is that the failure mode is simply that the assets load from the CDN as if there were no squid proxy. This is not ideal, but it's not so bad either.