Right, so it sounds like you are asking for this behaviour:

If /fopen -x is used to open a file exclusively, you want /fseek -l to create a cached index of lines in the file and to maintain this cache for as long as the file is open, to allow /fseek -l to jump to that line quickly. In addition, if the script uses /fwrite to write to the file at any time, this will clear the cache, requiring /fseek -l to rebuild the cache.

As this is a feature request, I have moved this thread to the feature suggestions forum.