(You didn’t mention what technique you are already trying in your gif.)
Right now, it sounds like you are trying to solve a general rigging problem. So I’m not going to answer an “mGear way” specifically. I always use extra custom rigging and POST scripts in my rigs.
This is a technique I frequently use for bags, straps, skirts and armor:
Use 2 nurbs ribbons. One carries controllers which follow the body. The controllers control a 2nd ribbon which carry joints which skin your bag.
- Make a nurbs ribbon with the number of spans matching the number of controls you want on the strap of the bag. I’d suggest something like 6 to 12.
- Attach the same number of follicles to the ribbon. So if you have 8 spans on the ribbon, attach 8 follicles.
- Under each of those follicles, add a joint and an animation control.
- Those joints will skin a 2nd ribbon.
- Make a 2nd ribbon. It can be a duplicate of the first. Or it could have a few more spans. I usually use the same number of spans.
- Attach a lot of follicles to this ribbon. Enough so that you can smoothly skin the bag. I would suggest 12 to 24. Also add or constrain a joint under each of those follicles.
- Those 12 to 24 joints will skin your bag geo.
- Copy the skin weights from your body to the FIRST ribbon. This ribbon is carrying your controls. Now your bag should follow your character, but the controls can still be animated, to shape the SECOND ribbon.
- If some of the controllers are bending or spinning too much with the body, I will sometimes add a group above the control and orient constrain them to other controllers on the rig. For example, the control near the bag could be orient constrained to your COG control near the hips, but still follow the position of the body skinning.
When I say follicle, I don’t actually use follicles anymore. I wrote about this and shared a script a while ago: https://gist.github.com/chris-lesage/0dd01f1af56c00668f867393bb68c4d7
Also, this technique is perfectly valid in Unity. Constrain your joints to the “follicles”, but keep the joints in a separate clean hierarchy and bake them before exporting.
To make this more “mGear”, you can store the ribbon skinClusters in your skinPack file. You could also use the mGear curve constraint instead of a nurbs ribbon.