Any way to fake/create a combobox.click event?

I have a series of 6 dropdowns that I am using as a datafilter for some XML data (about 1400 records). Each dropdown filters one attribute.

As the comboboxes are filled with values other than "All", I am filtering the contents of all the other comboboxes so that the only values in them are values present in the XML for the currently selected filter values. Basically, you cannot create a null search return, because the values disappear from the dropdowns if picking that would result in nothing passing the filters.

All this is working as coded with combobox.change events, and it is working fast enough to not be noticeable (less than half a second).

As far as usability and clarity, I would STRONGLY rather have this re-population of the dropdowns be done on a click event instead. The issue is that if a user picks a rare attribute by accident and then re-opens the combobox they just opened, very few options will be left in that combobox, including the one they may have meant to pick. So a user would have to select "All", to reset the contents, and the open the dropdown again.

If I could do this on a click event, I could filter the available options for that dropdown by the OTHER FIVE dropdown filters excluding the one that is JUST ABOUT to open.

The .open event also does not work well, because it waits till the combobox is fully open and THEN would repopulate. This is pretty undesireable as the user sees the values switch and if the combobox goes from one item to 10 items the size of the dropdown does not adjust, so it can cause pointless scrolling as well as look bad.

Is there any way I can get a click event on a combobox? I think I could do this in AS3 with the way the events travel by placing an invisible object above the combobox, but I don't think I can do that kind of trickery in AS2 (maybe not even in as3). This is for work and I am stuck with AS2 though.