"The Book" says:

Using external conditioned ground air simultaneously with aircraft air conditioning packs is prohibited to prevent chattering of the non return valve.

I know we are mainly discussing the 737 here, but that is all the Airbus FM says about it. It may be worded differently in the guppy manuals, but if you can point out to me where it says I can't use the packs with with the external air turned off but the hose still connected, I will bow to your misguided efforts to be a company man.