I do not see an obligation to distribute source to third-parties in the GPL. It declares the rights of those immediately downstream from the distributor to receive source code. Third-party rights are unclear. A third party is entitled to receive source from the second party in the direct distribution chain but how do these rights propagate upward to the original distributor?
I see no obligation to distribute source to parties that are not in the distribution chain. If you allow public downloading of binaries then each download is a distribution and source must be available for each download. If you distribute to specific parties then your obligation is to those specific parties but not to the public.
Section 10. Automatic Licensing of Downstream Recipients.
This section releases the distributor from obligations to enforce the GPL terms on third parties.
This may be nit picking but it is important to know what you obligations and rights are.