I really would not want to deal with the pressure on a hydrant unless absolutely necessary and, as has been stated, shutting it off again is going to be a real pain without the proper spanner.

Instead, what about carrying one of those hex key-type tools that opens a security hose bib. You know, the kind that omits the round handle, but instead has a square fitting inside the fixture that requires a key to open. I suppose you could pick up one at a plumbing supply store. In the city where I work, there are one of those security hose bibs about every 100 feet. They are not obvious, but are there if you are looking for them.

Craig.