I'd question what it is you're trying to do.... I'm not sure of any situation where you'd be fseeking to a byte position but wanting a line number.
If you use /fseek -l you have the line number right there.
Otherwise, with just a byte position, mIRC would not really be able to implement this any more efficiently than a script-- both of which would implement it inefficiently overall. To get the line number from a position in a file, mIRC would need to loop through each byte counting the lines as it goes. You could script the same thing, it's not nearly as convoluted as it sounds.
$fopen(fp).line might be an interesting suggestion though.
