Notes: For time signature higher than 4/4, each bar is spread out over 2 or more bars (e.g. 5/4 will be written as 3/4+2/4).
So would you need 4 pre roll bars to get a 5/4 count-in to fit ?
The only way I can see is have it work like the AutoHotkey script play the count-in with a metronome without playing bars -1 and 0 then start play at bar 1 OR any chosen bar to start the count-off as Reaper can do.
Unless you add RealTimeSignatures so then the count-in bars will be in the real time signature and can then play the correct 2 bar count-in.