I don't know of a way (even via scripting) to make the channel tab buttons flash whilst mIRC is the active application.
You can right click the channel window and check 'Flashing' to make mIRC flash every time someone says something in the channel. You can also go to ALT+O > IRC > Options and check 'Channel message' under 'Flash on:' which has the same effect except for all channels.
Also might want to look at /help /flash.
These methods will only activate when mIRC is not the active application however.
The way i was doing it was to halt the TEXT event and echo it myself into the window. So each time someone wrote something the channel became gray instead of red if you get me. Which I found confusing :\ .
What I should be asking instead is if I can override default mIRC modes.. ie when someone says something i want it to be displayed another way like <<$nick>> instead of <$nick>.