As crowns and miters are something worn on the head, some other kind of headgear, such as a helmet or a hat would probably be suitable for a piece with the additional powers of a man. While this might not work as easily for rook+man or bishop+man, these are also rook+ferz and bishop+wazir and are known by other names, such as dragon horse and dragon king. So maybe they could be represented in a different manner. Or all three might be represented by human face pieces like in the Superba set.
Even though chess pieces (unlike the pictograms used in 2d diagrams) are more than just the head or head cover, it is true that the head is the distinctive feature, and the bodies all look alike.
To indicate the KN move in a variant that also features the Amazon, you could leave on the cross. It is very unlikely there will be both a royal and non-royal Centaur in the same variant. Or replace the cross by a spike, to distinguish it both from a Queen and a King head.
The Jocly 3d pieces can provide some inspiration; the Crowned Rook there is a Rook with a crossless King head mounted on top. No room for confusion there; a Rook with a Queen's head would make no sense. For the Crowned Bishop I use a pontifical tiara. Both give pieces that distinguish themselves very well from other Staunton pieces.
Even though chess pieces (unlike the pictograms used in 2d diagrams) are more than just the head or head cover, it is true that the head is the distinctive feature, and the bodies all look alike.
To indicate the KN move in a variant that also features the Amazon, you could leave on the cross. It is very unlikely there will be both a royal and non-royal Centaur in the same variant. Or replace the cross by a spike, to distinguish it both from a Queen and a King head.
The Jocly 3d pieces can provide some inspiration; the Crowned Rook there is a Rook with a crossless King head mounted on top. No room for confusion there; a Rook with a Queen's head would make no sense. For the Crowned Bishop I use a pontifical tiara. Both give pieces that distinguish themselves very well from other Staunton pieces.