Not sure if there's a command for this. /window -d #channel doesn't work.

I'm experimenting with this Mosaico program which lets me specify auto tiling rules for windows. I set my mirc windows up to be tiled across entire second monitor. Anyway, since detaching mirc mdi windows to desktop window isn't retained on restart, having to do this manually is a pain. So yeah, a command I can call to detach the windows I specify would be great.

Hello,

if you want to keep window position after restart mIRC: you must save position of the window (or all) on menu right-click -> position -> save (save all), on "window" button (treebar or switchbar or titlebar).

you can use a static position before join channel with:
//writeini $qt($mircini) windows #MyFavoriteChannel 100,800,800,400,0,0,0 | join #MyFavoriteChannel