As someone who completes a lot (three gold over the last two weeks for my Grisette!), one of the biggest things judges pick up easily in comp beers is oxidation. A growler filler would let a lot of air in the bottles which could cause oxidation. It will also allow CO2 to escape which could cause you to lose a few points for under carbonation. Marshall from Brulosophy kind of took the idea of a growler filler and expanded it on. He used a growler filler, a stopper that would fit into the top of the beer bottle and a stainless wand to create something to kep the CO2 in and by squeezing the stopper, allow air to escape the bottle. If you could put something like this together, it would be better that just a growler filler.
The Bru Bottler
I used his contraption for many years, it worked fine and bottles were always still carbonated. However, plenty of entries received comments regarding low carbonation. You can try to avoid that by maybe turning the CO2 up 2-3 PSI 24 hours before bottling to add a little extra. A few years ago I upgraded to the TapCooler, a very simple and compact counter pressure filler. It's fantastic and allows me to fill bottles with CO2 first and then after filling, add a little more CO2 on top of the beer before capping.
Good luck in your first comp! If you don't get a great score, remember judging is subjective, so don't be discouraged. I had a NZ Pilsner score a 31 in a local comp last year. Decent score, but thought the beer was much better than that...sure enough, in 3 other comps it scored a 45,41 and a 40. As it turned out, the local comp lost a lot of judges due to a snowstorm, so they had a lot of non-BJCP judges judging beers who really did not know what they were doing.