I think the suggestion in the following thread is a better alternative...
https://forums.mirc.com/ubbthreads.php?ubb=showflat&Number=174955&page=1Basically a flag/option with the window command to automatically duplicate everything into the custom window (internall use aline i guess). Then maybe the addition of an on ^*:ALINE:match:@window:{} event (i know this kinda opens up to the logical addition of event for the other *line commands). There is still a slight change of a looping issue, but that could be resolved by adding an aline flag to prevent event triggering.
Hmm, i dunno, i still dont see a great deal of use for either suggestion, just throwing ideas around.