A 'frameless' backpack is not necessarily an unsupported or unstructured backpack. Many manufacturers will have various supports, including a backpad, that helps with structure. Additionally, a fully functional shoulder harness system and hipbelt system will still be in place.
Whether of not to use a backpack without a frame depends on the weight of the load. For most loads under 14 pounds, a frameless backpack works as well as one with an external or internal frame.
Fitting a frameless backpack still needs careful consideration to make sure it works with the users spine length. Many frameless packs will be manufactured with a broader size range for a pack: instead of small, medium, large, extra large (to correspond to spine length), there may be something like small to medium or large to extra large. Sometimes the shoulder harness offers adjustability to help fine tune to the wearer's spine length.
I can suggest a few frameless backpacks which I think are well implemented. Feel free to send a PM if I can help.