Well, sitting here eating a late lunch, and I decide to ponder the major point of the Hydra variant. I want multiple heads and I don't want to pay for them.
The way I decided to do this is to have a rack along one or two sides depending on if you want single or dual head capability. Each head has a dovetail-like slot on it, with a stop at the top. One on each side of the head. One is the hanger for storage. The other is active slide with power and data spring contacts. The carriage trolley needs either a solid solenoid or a motor drive on it. To pick up a head, the appropriate (open) trolley slot drops. It rolls over to the head and aligns the slot and holder. Raising the slot engages the head. When the head hits the positive slot at the top, we are aligned and should have a repeatable position.
There are other ways to do it. This method needs constant power. Perhaps an "expanding" dovetail would hold the unit better and not need power or a brake to lock the head in place through power dips, but also would need another actuator.
The trolley head holders are not meant to run constantly, just when the heads are being replaced. If a fourth axis is needed, such as for milling or inspection, it should be integrated into the head. Same for the polymer pumps and other material supply methods. Looks like I'll have to get acquainted with CAD again.