How to get lyrics melisma on S+A when T+B has the slur

I'm working on a closed score of S+A and T+B in voices 1 and 2 on each staff, with lyrics entered into the top staff but not the bottom (a fairly common occurrence for choirs). When the higher parts have a slur and need a melisma line to extend across them all, I can work it out by entering the lyrics into the right voice with underscores to extend a line, but when the slurs are in the lower voices and the upper voices have a single held note, I can't enter a line. If I cheat and make the top parts match the bottom in rhythm then enter a line, the lyrics look correct, but then when I change the value of the first note in the upper parts the line extends into the next notes instead of staying put. Is there a way to get the melisma line to cover the lower parts?