Ok am I a little confused but could you have not taken multiple shots and stitched them together to make the high resolution you are after.
@
Pookeyhead did a 40 image shot of his greenhouse if I remember correctly and when zoomed in you could see the individual water droplets.
I can't find the thread but sure @
Pookeyhead can assist.
Edit: @
Phal got in just before me.... written at same time on same page I think Etienne
I no longer have the image.. it was just a test after all, but the resolution was unbelievable. I use the IQ180 quite regularly, and the stitch from the D800E files FAR out-resolved the IQ180.. it was insane. It was also GIGABYTES in size, and brought a 6core machine to it;s knees, so not something you'd want to do every day. The problems of course are that you can't just take one image, and that curtails the use of moving subjects, crowded streets, or any set that contains movement.
Sometimes, you have to shoot medium format. Simple as that. 35mm is just not up to massive wall sized prints, even the D800E/D810... not if you have a discerning client anyway. Your local restaurant or whatever, will probably be happy with any old crap you give them, but a discerning client with definite print requirements will not be happy if you turn up with a small format camera. Surely the OP spoke to the client, and knew what they wanted though? Why not just hire the gear and mark up the final invoice? If they are so discerning about quality, surely they must appreciate that such a job will not be cheap.
It's quite likely that the client was confusing sharpness and image quality with file size. In which case, you could just interpolate to whatever file size makes them happy?
If they are aware of file size needs, then they'll probably not be fooled by an interpolated 24MP image from a 35mm camera

You can't add what's not there.
i just scanned in some large format 5x4 film. 48bit colour at 2400dpi , came to around 98megapixels and just over 500megs an image. You Nikon lightweights got nothing to moan about
It should have been FAR larger than that! A D800 TIFF at only 16bits is 205MB! and at 32bit, a D800 file is around 415MB.
When I scan a 5x4 neg on the Flextight it comes in at over 2GB, and that's at 14bit. At 48bit it would be a round 10GB in size! Besides... PPI means nothing... that's just print resolution. If you mean the dpi resolution of the scanner... that won't be optical. What kind of scanner you using? A flatbed? If so.. you're getting nowhere near 2400dpi resolution.What are the actual pixel dimensions of the resulting file? At 48bits a D800 file would be FAR larger than your 5x4 scan... somewhere in the order of 800MB.
You're doing something wrong
Also... why you scanning at 48bit? You do realise that level of bit-depth is simply not necessary and an utter waste of time?