Adding passive circuit to an Aguilar OBP-3, How to??

Yes, it's possible.

Take the output from the volume pots and run it to the third terminal of the tone pot, then run that to the input of the preamp.
Place your capacitor from the second terminal of the tone pot to ground.

I'll do a diagram if necessary...

 

Ok, connect the output from the tone pot to that switch.

 

I'll do a quick edit to that diagram so you can see where the tone goes.

These are 250K/500K volumes, before the preamp, so they will function in passive mode.

 

Oh line6man...you're the greatest. You will have some PM's soon when I get going on my project.

 

Cool. Thanks.

 

If you don't already have a preference for tone pot values, i would suggest 500K.

When a tone is all the way up, some signal is still being cut, so the higher you make the value, the brighter the tone will stay with the tone on full.

If you're willing to do a push/pull switch, you can bypass the tone control when you're not using it:
http://www.flickr.com/photos/jrkdesimone/3396717693/in/set-72157616015420655/

As far as capacitor values, if you don't already have a preference, i would stick with a 0.047uF.

 

The volume and tone controls as well as the tone bypass switch will work in both active and passive mode.

The tone bypass switch removes the tone control to maintain the regular tone without the extra capacitor in the circuit.

 

No, the push/pull turns the tone control off. When it's bypassed, the pot will not have any function.

As far as an LED, I'm currently working on a project with an OBP-1, and i was originally going to do almost the same thing that you want to do, but i was going to do a bi-color LED for active and passive.

According to the Aguilar diagram, the input of the preamp should be shorted out in passive mode. I'm not sure how it matters because the output is removed from the circuit, but i would rather not mess with it, so i am going to just do a power LED rather than active/passive LED's.
I'm doing a push/pull switch, but if you want to use a 3PDT toggle switch and use the third pole for the LED, that is definitely do-able.

Here was my original schematic for the LED, if you want to mess with it:

 

I've got a complicatedly wired Schecter that has custom Kent Armstrong dual coil humbuckers in it and OBP-3. It's wired V/V/B/M/T with each Vol being a Series/Parallel Push/Pull Pot, then and Active/Passive mini-switch, but no passive tone control. It was all wired up by Brian at Low End, but the wiring is so voodoo to me that I've as of yet been hesitant to try adding a passive Tone control to it as I don't really want to add yet another Hole/Pot to the bass.

 

Your bridge should always be grounded.
Ground the bridge to the nearest ground point.