Hand warmers and O2 absorbers are made of the same stuff... powdered iron and salt... basically controlled corrosion in a bag, with oxygen as the catalyst.
The buckets I use, have the big rubber O ring built inside the lid.... I'm using them for pinto beans without a bag.... they suck the lids down tight, and have been that way for over a year.
I don't see, or forecast any problems, as there is nothing ( even atmospheric changes ) that would pull air into the buckets.
I get the hand warmers on sale in the spring / summer at wally world ... since they perform so well, I expect one could use them in 55 gallon steel drums ( with lift off / locking lids ) also.
A few here were wondering about the O2 absorbers being good... remember that they get hard / stiff when they are exposed to oxygen ... when new, they are flexible and have powder inside.
I don't know about any gases given off by them... they use the available oxygen, but not sure if they give off any carbon dioxide.