Einfachste möglichkeit:
[autoit]Func __FFReadHTML($sMode ="body",$iFilter=0)
$sString = _FFReadHTML($sMode,$iFilter)
$sString = StringReplace($sString,"ä","ae")
$sString = StringReplace($sString,"ü","ue")
$sString = StringReplace($sString,"ö","oe")
$sString = StringReplace($sString,"&","&")
Return $sString
EndFunc
und dann statt _FFReadHTML, __FFReadHTML aufrufen.