One of the services the SA descriptors provide is query construction via column/relation mappings:

query(Foo).filter(Foo.bar == 2).filter(Foo.children.any(Bar.age > 2))

Not having those would diminish the utility of the ORM in user code- querying based on higher-level relations would be lost.

If combining descriptor functions is acceptable and the __eq__/__ne__ conflict can be resolved, I like Mike's suggestion about providing some get/set routing logic in the InstanceState. Any Trellis-managed attribute the ORM wanted to get or set could be directed to ob.__cells__[...].value; similarly, the combined Trellis/SA descriptor would maintain Trellis's __cell__ logic.

The collections may able to bolt on almost as-is. The collections package provides decorators that emit add/remove events for the ORM. If mapped instances enter and leave the Trellis collections directly, then it's pretty trivial. If they go in and out wrapped in something, then some custom code may be needed. The collection descriptor also has some logic in its __set__ that would need to be preserved. It does the same thing I would imagine Trellis does for collections: orchestrate events on foo.attr = [b1, b2, b3] or foo.attr = foo.attr[2:].