I personally have never heard this usage, but - since I have half a dozen British slang websites on my 'Favourites' list - I've just checked through them all. Only one refers to 'growler' and - believe me! - it has nothing to do with pork pies. It suggested that it was a form of rhyming slang based on 'growl' and 'grunt'. I'll leave you to work out for yourself what it referred to! (Perhaps it is connected to the 'pie' element which involves a rather different slang form that I have heard of before...I don't know.)
I have found some information for those of you that may be interested. A pork pie (the king of pies) is affectionately known in some circles as a growler (not just in yorkshire) We can only guess that the reason for this is either that they make your stomach growl after eating them or that some of them bite!