Is it possible to define a 'richtext' fieldtype as 'MEDIUMTEXT' in DB?
Defining your a plugin with it's own field type that extends RichTextFieldType might work (haven't tested), but your current code is slightly off.

The field type class/file name needs to be PluginHandle_FieldTypeNameFieldType before Craft will even recognize it.

Also, the ColumnType enum uses MediumText, not Mediumtext.

Really, what we should probably do is make this a setting on the field type because it does pop up every so often and there currently isn't a clean answer.