Since you can't modify the file, the only way would be to wrap it in a <frame> or an <iframe> and drive the searching and scrolling from JavaScript in the neighbouring/containing page.
Here's an example, which you can try out online at http://entrian.com/so-container.html
<html><head><script>
function go() {
// "line" is the <input> for which line to jump to; see the HTML.
var line = document.getElementById('line').value;
if (document.body.createTextRange) { // This is IE
var range = frames['log'].document.body.createTextRange();
if (range.findText(line)) {
range.select(); // Scroll the match into view and highlight it.
}
} else { // Non-IE. Tested in Firefox; YMMV on other browsers.
frames['log'].find(line); // Scroll the match into view and highlight it.
}
}
</script></head><body>
<input type='text' size='5' name='line' id='line' value='10'>
<button onclick='go()'>Go</button><br>
<iframe name='log' width='100' height='50' src='so-data.txt'>
<!-- so-data.txt contains the numbers 01-20 on separate lines -->
</body></html>
I've tested that in IE7 and FF3; I'd be surprised if it worked elsewhere without edits, but you never know your luck!
Obviously in your case you'd be driving the scrolling programmatically rather than via an <input> box, but you can see how it would work for you.