The game already has multiple access points for the Exchange. How hard could it be to adapt the same function for mission start points? Each holodeck would be the same door copied and pasted at diffident locations.
The Foundry should have an option that would let players start holodeck missions from any holodeck they have access to. This would go a long way towards making the mission feel like a true holodeck program and less like a world event.
Foundry Character slots can be bought on the character selection screen with zen points. Foundry Project slots can be bought on the project selection screen with dilithium.