Found some interesting info. It turns out --no-cache with wget sets headers Cache-Control: no-cache and Pragma: no-cache, so you could use the same headers within Pulp. The headers would need to be hacked into the Pulp code somewhere, perhaps in this file: https://github.com/pulp/pulp/blob/f231a283e5d377d56b6c2061df9e2d3d1bc81cd9/server/pulp/plugins/util/nectar_config.py#L98-L99
Hopefully though you can get your proxy fixed!