I don’t think you can be a gloryhunter supporting a lower league team, but I do know quite a few people who are Medway/Kent born and bred and support Millwall or West Ham with no family links to south east or east London. I’ve always found that strange - especially supporting millwall - who for most of the lives of these guys have either been in our division or just above us, so not sure why they wouldn’t go and support their local team rather than trek 30 miles or more to watch someone in the same division or division above? West Ham are obviously generally in the top flight but I if I were to not support my local club and support a London team, I’d go the whole hog and support a good one. The only reason I can think of is that most of those from Medway I know who support west ham or millwall are white van man types, so maybe support them for their wide boy reputation? For example, most of them refer to West Ham as West ‘am or the ‘ammers, even though they don’t speak like that normally.
The Millwall ones are the worst imo. Many seem to think it makes them "well 'ard".
Living in SE London for 40 years the place is full of blokes wearing Millwall hats and tops to look tough but never go to games (I've actually asked a few and they rightly looked embarassed).