Continuous Mana is found in GURPS Thaumatology alongside Non-Mages Casting in Lower-Mana Areas. Instead of fixed "values" for mana there is a gradation of -10 to +10 to base spell skill.
It has a suggestion of allowing non-mages to cast anywhere at -6 to skill. Combined with the x10 cost for NMC, it's pretty inhibiting.
An interesting effect is that since mana level effects Item Power and the range of normal mana (-2 to +2) under this mechanic, it is possible for a magic item to not function in normal mana even if created in normal mana. Conversely this means the Item Power will increase in areas above normal mana.