More fantasy than anything else. Normal fecal matter would block up that small hole in the tunnel plug until things got real uncomfortable. The only way a small hole will function
is for the fecal matter to be in the form of a liquid (diarrhea). Therefore a diaper would be the required wear just as if one were incontinent. If the tunnel plug were larger then
the passage of waste matter may be easier but the insertion and wearing may well might be stretching the body limits. The end result (pardon pun) is that the tunnel plugs
create a semi incontinent state in which the individual does not have control just like wearing a catheter. Definitely a diaper wearing state or a similar situation to a untrained
puppy dog.
About that dryness state - have no fantasy experience here but would presume that with the anus open to the air that the anal lining would be deprived of passing liquid matter
to lubricate the canal thus preventing a rash or irritation. It is not nice to fool around with mother natures plan of waste evacuation ! Of course the plug it self would be protecting
a small portion of the anal canal but upstream of that point unless the flow is in a liquid state - constipation is going to be an issue ! Air dried poop does not flow freely and would
tend to do another sort of plugging the system !
A ha but the very thought - the madness of being a self induced incontinence - being a walking talking pooping machine with no control and perhaps even whistling a tune
through that pipe up your ass ! LOL !