If you've got only one popup window you can use null as a target and test will take the first popup:
waitForPopup | null
The other option is to get dynamic part before popup opening. It is very likely that the dynamic part could be retrieved from the page. If so you can get it using storeEval, and than use like:
waitForPopup | javascript{'static'+storedVars['dynamic']+'static'}
If you can't store the dynamic part please provide an html of your page or only the part where the dynamic part mentioned.
I see that theoretically it could be possible to get all the names of your windows and than to use pattern in a loop to get the one.
Also (theoretically) it is possible to expand default waitForPopup function.
But the second way and especially the first are much cheaper.