The Romans have enough administrative efficiency to pull off a levee en masse already. They've only held off on doing it because of the memory of the Time of Troubles, when they saw that mass levies tended to cause horrific civilian casualties because the enemy can no longer distinguish soldiers from civilians.Levee en Masse is a bit anachronistic. The Regimental Cantons system maybe as a natural evolution of the Theme System as a force of conscription for all able bodied men rather than recruitment and upkeep. But even then that's still a century removed institution and one unique to a nation who by necessity needed a proportionally large army to even survive, alone compete, in the European world of the 18th century. We're in the midst of the 30 years war era of warfare, and that shit ain't pretty or efficient.
Analogy to Prussia is apt for the Empire, in that it is surrounded by hostile powers on multiple sides and is at a quantitative disadvantage. If the standing Imperial Armies start to falter, I can see Demetrios implementing modern style mass conscription to make up.