I think you could use the following JavaScript to track the last-focused textbox:
<script>
var holdFocus;
function updateFocus(x)
{
holdFocus = x;
}
function appendTextToLastFocus(text)
{
holdFocus.value += text;
}
</script>
Usage:
<input type="textbox" onfocus="updateFocus(this)" />
<a href="#" onclick="appendTextToLastFocus('textToAppend')" />
A previous solution (props to gclaghorn) uses textarea and calculates the position of the cursor too, so it may be better for what you want. On the other hand, this one would be more lightweight, if that's what you're looking for.