They can be.
I broke a saddle bolt on a road south of Davenport, Illinois on a long ride down through the Mid-West of America.
I stood at the side of the road for a while trying to figure out how to re-attach my saddle with cable ties but could come up with no solution. And then a gentleman, whom I had just been talking to at the local gas station, walked by and asked if I had a problem. I explained the situation and he took me to see his friend, the retired local sheriff, who was passing his very late autumn years renovating bikes for local kids.
Short story ... he rummaged around in his garage and eventually produced an "it'll do" fix which saw me through to Muscatine and a proper bike shop where I bought a new bolt and a spare and kept the bits provided by the sheriff.
By the time I'd got to New Orleans both seat bolts had gone and I was back to using the sheriff's "it'll do fix".
As a note ... I was credit card touring with only a Carradice Camper Long Flap and Bagman Support. I'd spoken to Sheldon Brown about this arrangement before setting off and he seemed to think it wouldn't be a problem ... so, Sheldon, you weren't right about everything.
These days I use an Arkel rack bag rather than Carradice saddlebag.
I also don't use Brooks saddles which may also explain the Carradice failure.
Anyway, I think it's just an M7 bolt you need ... and well worth carrying as a spare on longer tours.