As far as I know it's impossible to do with filter, as it's designed to output text, not to modify it.
A $fline() loop with /rline replacement is a fairly reasonable solution.
while $fline(@win,*áôäë¬çíšÅ*) {
rline @win $ifmatch $replace($line(@win,$ifmatch),áôäë¬çíšÅ,$chr(32))
}