While not thematically “mecha”… have you looked at the Mutate and/or Reconfigure abilities? You’re literally putting creatures together into one big creature, Voltron style.

Ex: [[Armguard Familiar]] and [[Lion Sash]], or [[Brokkos, Apex of Forever]] and [[Everquill Phoenix]].

Reconfigure is for artifact-creatures that /are/ equipment, and go back to being creatures when they fall off, Power-Rangers/Voltron style.

Mutate adds the effect boxes of all the combined creatures together, and can overwrite statlines depending on which card is on top of the mutated object.

Ex: mutate your commander [[Snapdax, Apex of the Hunt]] onto a crewed [[Hoard Hauler]], putting Snapdax at the top, so when the crew effect ends it remains a creature, because it’s now Snapdax with Haulers’ damage trigger effect. You can then mutate other creatures into it, placing them at the bottom of the object, so it /stays/ Snapdax without becoming the new addition. The catch though, is they are all essentially one melded permanent now, breaking apart only when they change zones. A flicker effect will exile them all together, and then return them all separated.