Like the one on the intro of "IF" by Bread.
 
There's countless speculation on the net as to how they did it including a processed studio effect not easily done live. None of the videos I've seen of Bread playing "IF" ever sounded right, so this may be true.
 
To me it sounds like a pedal steel guitar thru a double layered wah, one fast, one slow.
 
Here is a convincing stab at it using Sonar and Guitar Rig.
 
1. Insert pedal steel synth and MIDI track, or record real guitar/pedal steel audio track.
    (I used a free Pedal Steel sound font found online and Dimension.)
2. Load GR in the synth's FX Bin and use only a Wah filter.
    (I used the Cry Wah)
3. Create a rising saw blade envelope for the GR volume pedal like the one in the pic.
4. Add a little StudioVerb2 or other verb to the mix, and...
 

 
"wavey-wah comparison.mp3":  https://onedrive.live.com/?cid=96E82842C838B785&id=96E82842C838B785%21250
 
1st bit is mine followed by the original intro. The sound is very close. Enjoy!

I just found a most interesting answer to the puzzle. You'd never believe it if told by anyone else but the man himself.
 
When “If” was recorded, the track was enhanced by a mysteriously quavering sound that gave the song a distinctive sheen.  “That was created by two Moog synthesizers,” Gates reports.  “Paul Beaver came in and set them up – I played a plain old Fender Telecaster guitar through a voltage-controlled amplifier, and he put that into these two oscillators that triggered each other in random fashion.  When we were all done he said, ‘I hope you liked that and got it on tape, ‘cause I could never do that the same way again.’”
http://www.albumlinernotes.com/Bread_-_Retrospective.html
 
No wonder they couldn't pull it off live. And all that tremolo and wah pedal speculation dashed in one sentence.

Really nice effect though. Your method is definitely more useful and adaptable - Thanks for sharing!