I did wonder if it could really be effective, I've just seen so many comments mentioning it and I like to run all sorts of chicken ideas past the BYC community. Here in the UK people are buying live fishing maggots (white ones, before they are dyed which is common here for fishing purposes). Here are just some of the posts I have seen... I appreciate these are not the results of a scientific study.

I think in most cases people are trying oil and massaging as a first port of call or alongside the 'maggot treatment' so more likely to be that helping I would've though. Intriguing anyhow!
"Yes we’ve used it and it works. They need to be white fishing maggots though not dyed. It worked pretty quick for us. I’d give a small handful and see how she goes."
"We used white maggots for one of our girls who's crop hadn't emptied in over a week, the vet said she hadn't got long left and her crop not emptying was down to a tumour, we fed maggots and the day after her crop was empty and she's still going strong!"
"i think the lump is reducing!!! Just woken her to give her some more..... she’s had about 60 today!!!"
"I used the maggot thing too. It worked a treat."
"When I have a crop problem I normally bring the poorly hen indoors & give her half a handful of maggots in a bowl then pop her back outside with the others & throw lots on the ground for them all to eat! I have found maggots work wonders for crop problems. "
"Yep I've used this method a few years ago when one of my girls showed signs of impacted crop. The maggots eat their way through the 'matter' before being digested."
"Worked for me i brought the maggots from a fishing shop nearby."
"If one of your hens has impacted crop i would really suggest the maggots worked well for me. "