You don't say why you need to estimate the pressure drop. Is it to design the appropriate tower? Or is it to estimate the pressure drop in an existing tower?
Like yourself, I was once saddled with such type of towers - they were strippers. I had to modify one tower for additional capacity, keeping the same, simple horizontal "baffle" plate as the basis. I estimated the pressure drop from empirical field data as 1" WC per pass. I remember that I got very close to the actual result on the finished product. As I recall, the vapor velocity through the falling "sheet" of liquid was estimated at approximately <5 ft/sec. My tower was working at 5 psig and had 12 plates.
I have never succeeded in finding a relationship or method of calculating the pressure drop for such an operation and all the people I asked at the time just estimated it from previous experience.
Sorry I can't give you what you probably are looking for, but I hope the above helps you out.