This wouldn't be possible without an overhaul of the buffer window
mIRC doesn't know what part of a line (if any) is the timestamp, much less the nickname. Note that scripts often do /echo $timestamp instead of /echo -t, and a nickname can be in any form due to theming. mIRC has no way of discerning the two unless it did some kind of weird regex match on every copied line, and this would only be able to recognize the timestamp with any certainty.
Column selection would be better.
