Yeah, there's no exported way to get meteors to appear immediately. It can be done with mem hacking by setting two flag bits and resetting the unit's internal timer though.
The flags are at offset 0x44 in the unit record. The flags in question are 0x4000 (DisasterNoFirstWarn) and 0x8000 (DisasterNoSecondWarn). Then set the timer value to something like 0 or 1 (0 might not work since the timer might only fire when it counts down to 0 and not when it already is 0). The timer is at offset 0x3C. (I think, it's been a while). Both fields are 4 bytes long.
There's also some other timers you might come across, such as stickyfoam at 0x54, EMP at 0x56, and
(ESG?) at 0x58. All these timers are 2 bytes long. I believe there may be some inconsistency with how these ones behave when setting them to 0. Some will stop and the effect wears off, others will wrap aroung to 65535 and last a long time.