Found a workaround. You can't make any arbitrary font bold or italic and you can't use DrawableFont at all. but just using the Font and FontPointsize properties works as long as you format the font name like ImageMagick expects:
using (var working = new MagickImage(@"c:\temp\filename.jpg"))
{
// Draw text
working.StrokeColor = new MagickColor("#FFFFFF");
working.FillColor = working.StrokeColor;
working.Font = "Arial-Bold-Italic";
working.FontPointsize = 24f;
var metrics = working.FontTypeMetrics("Why hello there!");
using (var piText = new DrawableText(10, working.Height - metrics.TextHeight - 10, "Why hello there!"))
{
working.Draw(piText);
}
}