So I stumbled across this rather by accident and by just wondering if it would work and well my results show that it did, whether this is just a very coincidal coincidence remains to be seen but for me this did result in a Ghost Baby!
You will need a Sim that is skilled in Logic, you need the Chemistry Table from Generations. Get your Logician to learn the Ghost Potion and then create it, I did this with a Sim who had maxed Logic before Generations and do not know the skill level needed. Make two of these potions. The effects cause the drinker to act just like a Ghost for about three hours, it wears off eventually and they return to looking and acting like a normal Sim. You need a female and male sim to throw this potion and turn them into temporary Ghosts. Then continue with a Try for Baby until you hear the jingle.
Three days later I got my result, I had actually forgotten I'd done the Ghost Potion and so when the new baby came out of the Hospital and I had the shock of my life, the first and only Ghost Baby in my Legacy!