After pondering the question for most of the day, I am reiterating the existing policy. Structural customizations are not possible outside of the bounds of a canon-to-canon refit kit.
Therefore, if for some reason you had, for example, a primitive Ostwar, you would not flatly not be able to perform a customization which changed the design from to a Royal Ostwar spec. The Royal Ostwar requires Endo-steel, while the primitive version uses standard structure. You would be able to perform a customization to upgrade the engine, cockpit, and armor as per the standard rules, but as you cannot change the internal structure, the "Royal" spec is therefore simply out of reach. Customize it elsewise how you'd like, but an Endo refit is out of bounds.
The sole exception was, and remains, a fluff-supported refit kit that upgrades internal structure to a different type. An example would be the refit kit for the ENF-4R Enforcer which upgrades to the ENF-5D model. However, as a reminder, refit kits are only produced for TRO 3025 Mechs which upgrade to foundtech models in the 3050s. The Ostwar is, technically, out of production ("extinct") between 2790 and 3075 when it re-enters production, and therefore no refit kit could exist for it to bypass the existing limitation.
This is really the sole hard boundary in this campaign when it comes to customizations, and on reflection I am sticking to my guns on this one.