you could set ur .ini file differently so you dont have to worry about line length limit something like,

[avoice]
<#channel>.num=<num>
<#channel>.1=<address>
<#channel>.2=<address>
<#channel>.3=<address>
.....
<#channel>.<num>=<address>

<num> holds the total number of addresses so you can loop through and check each one. <address> is the addresses of people to avoice. <#channel> is the channel name.

i agree that it would be nice if the 900 char limit was removed from mIRC, but that is a limit placed on mIRC by a call to an OS function, wsprintf() i believe is the function that places this limit.