Evan, I don't have any experience with them, but did find the following thread over at AGF. It doesn't have a lot of information, however:

http://www.taylorguitarforum.com/forums/showthread.php?t=197442

Another choice for adding volume control to a guitar with an active pickup, but no controls, is the Schatten Black Box:

http://www.schattendesign.com/blackbox.htm

Brian;

Thank you very much. I was thinking of the V cable for my Rider, but in talking with the folks at Sweetwater there seemed to be some question as to how well the cable worked with passive systems. Also, the reviews you pointed to suggest that the V cable is more-or-less an off-or-on product. The reviews also suggest the Guyatone Speed Cable, which looks it might work for my application. I owe you one.

Hey Evan,
I have a guyatone speed cable -it does work with my k&k passive pickup. i don't find it adds noise, but it also only works from about full blast to halfway before you can't hear it, so it's a pretty rough, crude control that's hard to adjust when you just want to be a bit louder or softer. I've never tried it with an active pickup since I don't have one! I liked it mostly to just short out the cable or turn the guitar 'off' while still plugged in. Happy to answer any questions, or I could even mail it to you to try out - I mostly play wireless now and rarely use it.
-Penny

For a passive system, I would be tempted to get something like the Pickup the World Power Plug. It not only adds volume control, but is a battery powered preamp. Here's a link:

http://www.pick-uptheworld.com/preamps.htm

Penny; Thank you for the information. It's beginning to look like the available volume-control cables are essentially on/off units. I received the Planet Waves on/off cable from one of Michael's many contests and it is perfect at what it does. My Rider has a passive system and I wanted what you suggest--something that enables me to fine-tune my volume without having to lean over the amp, often in the dark, and make adjustments during play. I appreciate your gracious offer of a loan, but prefer not to borrow. If you are thinking of selling the unit, we could PM a bit and maybe reach a deal.

Brian; Thank you for yet another option. The power plug is an interesting possibility but has me locked into another accessory that requires a battery. The Rider RMC pickup is sweet, the mini-tuner fits in the sound hole very unobtrusively, and now all I need is some clean, simple way of controling volume. The Taylor and Guyatone both looked good, but, as Penny notes of the Guyatone, both may be too crude.

Evan, generally, it's not a good idea to try to control the volume of a passive pickup with a passive volume control. In most cases, sound quality takes a big hit. The V Cable or the Black Box are really designed for active systems that don't have volume controls.

The Power Plug seems to me to be an excellent choice if you want to add volume control to a passive pickup. Yes, it requires a battery (maybe even two batteries). However, the batteries should last a long time unless you're plugging in a lot.

I wonder if that Mi-Si Simple Jack could be installed in your guitar? I think you can get an onboard volume control to work with that, or you could then use the V Cable or Black Box.

Brian;

Thank you for thinking this through with me. I've thrown the issue to Joe at Blackbird and will continue to ponder the options. One such option might be just to count my blessings and plan on living with what I have, wearing it out, or doing without.

The RMC is a passive system as previously noted, but it is a very hot one so the signal that comes out is nearly equivalent to an active signal. Perhaps that will help if you go with some of the above options.

As the Guyatone seems designed for passive systems, it may be the best bet. Still pondering.

Evan,
I don't know anything about them, but would a volume PEDAL work for you?

I've had a chance this evening to play around more with the Guyatone Cable and my K&K equipped Cargo and the AC33. I used the looper and played at various spots on the volume control - it really does control the volume- from nothing to full tilt (the level of a regular cable, NOT louder -there's no boost) and levels in between. I think my ealier comments about it being more all or nothing was more a reflection of my hearing and the fact that the guitar is acoustic and I hear both it and the amp while I'm playing - when I play back the loop I can hear it better, and it does not boost the signal any louder than the volume of a regular cable, it just can make it softer or shut it off.

It works nicely with the looper to lay down a rhythm track at a lower volume, then push it to 'max' and play lead at the louder level. It's still rather crude, being a slider, and passive, but it does work. I think the key would be setting the levels on the amp and the midway point on the slider so you have what you want to work with. Whether it would work for your pickup and needs is another story, but I actually can see where I might find it useful, esp when using the looper. Hope that helps a bit.

-Penny