Does the path used by the old Wareham-Waterloo stopper still exist? If it does an 8car or maybe a 12 car 450 could be used between Weymouth and Waterloo (SDO would be needed at some stations). Could any services that stop at many of the stations on the route be run with 450s?
The previous Wareham - Brockenhurst stopper was combined with the Waterloo to Southampton semi-fast when the Weymouth went 2 tph. The 'stopper' now runs as a Waterloo to Poole service, which covers the two different roles and basically runs in the existing paths. Over the course of the day it already runs as any of 4,8,12,5, or 10 car as appropriate to the required capacity at the time.
450s and 444s can and do appear on any service between Waterloo and Weymouth and points in-between, nothing you are proposing doesn't already happen so I propose you give up...