Is there any means by which to limit the total quantity of products ordered via the Ubercart module? -> yes, as is default resident (disabled in Workflow rules)
This is particularly important for overseas shipping, which (from Australia) involves different rates for different quantities (weights).
This functionality is easily achieved with Wordpress (max quantity plugin), or with Drupal 8 (Commerce Extended Quantity module). But it seems equivalent functionality is not available in Backdrop CMS, which given that, imo, Backdrop is the better system, with super-easy, intuitive admin, is my much-preferred CMS.
update - I'm not referring to max-quantity per "small package" that is a default field on new-product entry. I mean max-quantity per order, which rejects attempts to add-to-cart beyond the defined limit.
update 2 - whoops, forgot that backdrop is integrated with rules -- was thinking D8, which isn't.. seems rules can do the job, perhaps similar to this D7 solution https://www.drupal.org/node/2922011
update 3 - wow, max quantity is already a (disabled) rule in workflow. Brilliant. Excellent.