Are hot-air balloon pilots ever required to file flight plans? (can they file?)
As a balloon pilot: no we do not. We do fly regularly in classes G, D, and E and will occasionally wander into class C; but it is very frowned upon...and you better have an aircraft radio or at least call the tower to let them know you are there. Of course balloons always have right-of-way so if you see us get out of the way; we can't control where we're going.
A quick look at the FAA rules regulating hot air balloons makes it sound like, in the USA, they can enter any sort of airspace provided they can do it safely. So, theoretically, they could file a flight plan or at least alert local ATC to their activities.
All of that being said, having been involved in hot air balloon operations before, I think it would not be terribly feasible.
Firstly, flying a hot air balloon outside of Visual Flight Rule conditions (being able to see several miles) would be extremely dangerous, so you wouldn't need to file a flight plan for that.
Secondly, you generally only want to fly a hot air balloon in very sparsely populated areas (Class G airspace), and while you could have a flight plan there really isn't a need. A more congested area (Class B or Class A) would be extremely dangerous to navigate in a hot air balloon. It's pretty hard to get out of the way, since controlling where you are going is...iffy sometimes.
So, in summary, yes it could be done, but in the vast majority of cases there's little need to do it. Other than to just alert local ATC that you will be in the area.
ICAO Standards do not differentiate between hot air balloons and other flights when it comes to flight plans. The following is from Rules of the Air (Annex 2):
A flight plan shall be submitted prior to operating:
a) any flight or portion thereof to be provided with air traffic control service;
b) any IFR flight within advisory airspace;
c) any flight within or into designated areas, or along designated routes, when so required by the appropriate ATS authority to facilitate the provision of flight information, alerting and search and rescue services;
d) any flight within or into designated areas, or along designated routes, when so required by the appropriate ATS authority to facilitate coordination with appropriate military units or with air traffic services units in adjacent States in order to avoid the possible need for interception for the purpose of identification;
e) any flight across international borders.
Note.— The term “flight plan” is used to mean variously, full information on all items comprised in the flight plan description, covering the whole route of a flight, or limited information required when the purpose is to obtain a clearance for a minor portion of a flight such as to cross an airway, to take off from, or to land at a controlled aerodrome.
As mentioned in the note, a flight plan may simply be an abbreviated flight plan, which is submitted over the radio. Such a flight plan would typically just include identification, type, flight rules, persons on board and intentions.