I was able to achieve this by using BrowseProjectFilter:
BrowseProjectFilter browseFilter = new BrowseProjectFilter()
{
Name = "File Geodatabases"
};
//allow FGDBs to be selected
browseFilter.AddCanBeFlag(BrowseProjectFilter.FilterFlag.FGDB);
//exclude everything else
string[] names = Enum.GetNames(typeof(BrowseProjectFilter.FilterFlag));
foreach (string name in names)
{
BrowseProjectFilter.FilterFlag ff = (BrowseProjectFilter.FilterFlag)Enum.Parse(typeof(BrowseProjectFilter.FilterFlag), name);
if (name != "FGDB" && name != "Container" && name != "Workspace" && name != "Database" && name != "DontBrowseFiles")
{
browseFilter.AddDontBrowseIntoFlag(ff);
}
}
OpenItemDialog dialog = new OpenItemDialog()
{
Title = "Select Output FGDB",
MultiSelect = false,
BrowseFilter = browseFilter,
};
dialog.ShowDialog();