Conventional wisdom says "no", because breweries are places where it's quite possible to get hurt, and non-employees aren't covered by insurance.
Still, brewers can be "the hell with rules" kinda people. I know that about five years ago, I hurt my back while working as the brewer in a brewpub in Virginia (lifting a keg wrong, ouch!), and wasn't able to work for a couple of days. I had to come back to work before healing fully, though, because I had to make the beer! A local homebrewer who I had met at a homebrew club party offered to come in and help me with the heavy lifting (he did shovel a lot of grain!), and together we were able to brew a bunch of beer and have a good time doing it.
I've been a production brewer in a couple of different microbreweries as well, and can testify that it's nice having company while working a brewing shift solo!
So, without expecting much, go ask your local brewers if you can come hang out and learn something. They might surprise you!