In line with history, I suggest to add the option to cheat the treaty displacement limits.
As this should not be automatic options and there should be a downside, I suggest the following:
Cheating limited by XX% of the nominal displacement limit (e.g. 25% for 10.000 ton limit)
"overweight" ships would be more expensive - assuming that the "maskirovka" costs something extra
The higher the excess weight, the higher the risk some other power will find out with political consequences (increased tensions) and impact on prestige of the player. The player could have the option to either scrap the ship(s) or face the consequences.