dropdown list for subcategories in center box

I hope this is an interesting question. I have a fairly large site, over 20000 products when finished, and have some categories that are fairly deeply nested. In one category the subcategories are shown as alphabetic groups, which may not offer the best solution but due to the number of products it is the only solution I have come up with. What I would like to be able to do is if the customer 'hovers' over a subcategory listed in the center box, say ABC, he/she would see a drop down list of the products/sub-subcategories that are in that particular subcategory. Hope I have made myself clear. Probably clear as mud! Anyway, is there any way to achieve what I am wanting to do? Perhaps with the stylesheet?

I appreciate your inputs!

I have decided to drop this request. Putting the categories based on
the English alphabet does not really work well with the international,
and non-English speaking, customers. So, everything is back up front. Going to be a lot of subcategories on the page (100 or so) but
that can't be helped. Thanks to anyone that might have looked at
this question.

this mod may be of use for filtering many subcategories
http://www.zen-cart.com/forum/showthread.php?t=161143