I remember reading that there is some init for the IIfx to use a HD20 (you had to take it apart and connect it directly to the logicboard)

Cant for the life of me remember where though...

I wouldn't try connecting it to a unsupported mac. i do remember the guide for using one with the IIfx essentially said "it's a bad idea and may void your warrenty, but will let you get your data"