This seemed to do the trick:
var loadedAssemblies = AppDomain.CurrentDomain.GetAssemblies().ToList();
var loadedPaths = loadedAssemblies.Select(a => a.Location).ToArray();
var referencedPaths = Directory.GetFiles(AppDomain.CurrentDomain.BaseDirectory, "*.dll");
var toLoad = referencedPaths.Where(r => !loadedPaths.Contains(r, StringComparer.InvariantCultureIgnoreCase)).ToList();
toLoad.ForEach(path => loadedAssemblies.Add(AppDomain.CurrentDomain.Load(AssemblyName.GetAssemblyName(path))));
As Jon noted, the ideal solution would need to recurse into the dependencies for each of the loaded assemblies, but in my specific scenario I don't have to worry about it.
Update: The Managed Extensibility Framework (System.ComponentModel) included in .NET 4 has much better facilities for accomplishing things like this.
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