Can anyone point me to a Mellotron-like patch in any of the SONAR Platinum VIs?  Thanks!

You should have the (old) Rapture.  Go to patcharena.com and download Mu3uem.  It has several mellotron sounds, I believe.  If not, poke around for one.  There are thousand of presets available for Cake synths.
 
http://patcharena.com/downloads/index.php?subcat=83&ENGINEsessID=e420a05e5832f940e8cef5460c9a7784
 

Not sure what it was on my PC, but I had to remove the Muz3um patches. Whenever I tried to load one, Win10 would try to access a non-existent drive. After clicking out of that popup, the patch would load OK.
 
BUT...
 
That was the Muz3um patch set that had been converted for Z3TA+2, and I was getting the faults using Z3TA+2, not Rapture Classic.
 
If you have Kontakt, here's a free Mellotron VI for it:
 
http://www.premiersoundfa...lotronpremier-p-3.html

My first thought would not be Z3ta+ or z3ta+2 for Mellotron sounds. I did just look and the converted and original Muz3um programs are bundled with Z3ta+2. They seems to work OK for me. But I still would look elsewhere.
 
There is a set of Mellotron samples which will load in DPro or Rapture (I prefer to use Rapture). Another user created sfz files. I believe the links are dead. Maybe b rock will reup them somewhere. You could PM him about it. I can also send you the sfz files if you PM me. Or you can make your own. The files are pretty easy to create. The thread is here http://forum.cakewalk.com/FindPost/1410821
 
RPro has a few tron patchs. I am not sure if they are part of Rapture Session.
 
The X2 Content Club Dimension Pro Holiday Pack includes a tron boys choir.
 
It might help to know what Mellotron sounds you are looking for since the instrument used a variety of different samples.
 
 

I can't remember if this was an expansion pack but in Dim Pro I have "Digital Sound Factory Classic Keys" In the folder "Electro Mechanical" there are Mellotron Strings and Choirs. Looks like I have an issue with the location of the multi samples with these patches so I can't comment on their usefulness, but if you have them they could be your best bet.
 
Again in Dim Pro under "16-Vocals"  the male choir is not too disimilar from a mellotron sound, the femail choir less so.
 
The Rapture Pro Factory Library has
"Choir with Mellotron" in Orchestria Stacks
"Mellotronish" in the strings - This is worth checking out.
 
 
 

The Mellotron sounds I'm after at the moment are the gritty strings used by e.g., Moody Blues, Genesis, etc., as well as the Strawberry Fields style flute.
 
I have the full Rapture in my kit (sans expansion packs), so I'll snoop around in there.
 
I saw reviews of GForce M-Tron online and others, but I didn't want to purchase what I might already have, or near enough.
 
Thank you all for the pointers! 

M-Tron Pro is very good. You can hear several instances of it in the track linked in my signature.

If you have the full version of Kontakt the Trons in Hollow Suns Prog Pack are wonderfully raw and powerful sounding.