This is how we do it:
private static void OverrideDefaultFont(string defaultFontNameToOverride, string customFontFileNameInAssets, AssetManager assets)
{
//Load custom Font from File
Typeface customFontTypeface = Typeface.CreateFromAsset(assets, customFontFileNameInAssets);
//Get Fontface.Default Field by reflection
Class typeFaceClass = Class.ForName("android.graphics.Typeface");
Field defaultFontTypefaceField = typeFaceClass.GetField(defaultFontNameToOverride);
defaultFontTypefaceField.Accessible = true;
defaultFontTypefaceField.Set(null, customFontTypeface);
}