An important assumption of OD&D, in my mind, is that unless otherwise stated things work relatively realistically. This is the wargaming context at work. In other words: I don't think the rules suggest that a thrown crossbow bolt should be considered a weapon.The rules don't suggest this, but should it be allowed?
In keeping with the assumption of "relative realism" Brendan alludes to, the limiting factor to a crossbow bolt thrown by hand should be its short range and imprecision. You can throw a dagger, so why not throw a crossbow bolt? Essentially, it becomes a dart, close in size to a lawn dart, actually. Lawn darts are illegal in the US specifically because of their potential lethality, so it's not that far-fetched that you could use one as a weapon. Later editions of D&D certainly didn't seem to think so.
The main reason you wouldn't throw a bolt is that firing the bolt from an actual crossbow improves your range. Any thrown object is limited to fairly close quarters in D&D; short range is 10 feet, which is also melée range, so you gain little by throwing the bolt instead of stabbing with it. If you have a house rule that ranged weapons fired in ways they weren't designed for are at half range, it's even worse: 10 feet would be medium range, 15 feet would be long range. (I'd probably use this house rule myself; it could apply to firing bolts from a regular bow as well.)