Modular synths with EXT-IN as effects processors?

The answer is yes, but you'll always get varying degrees of processing, depending on where the inputs take you in relation to the signal flow. Modulars are designed to integrate with other equipment (and i know for a fact that Doepfer makes a module specifically for this purpose) and can contain many audio processors. Otherwise, with a Minimoog (i know, not a modular) for example, you're looking at only being able to access the filters, and potentially the ADSR. Some synths (Korg MS-20, a semi-modular) have a pitch-to-CV convertor for triggering the oscillators! It's well worth looking at a lot of manuals for various manufacturers.. check this page for starters:

http://www.sfu.ca/~gotfrit/courses/247_2001/w5_synthesis_02.html

Pretty basic, but gives you an idea of what you can use for audio and what requires control voltage.

Also, here's the module overview page for Doepfer:

http://www.doepfer.de/a100.htm

You can go as far out as your brain/time/wallet (especially your wallet!)
allows.. You get amazingly forward-thinking sounds, but remember: no presets! It's a slippery slope, but well worth the investment..

 

I'm pretty sure the ex/in's on the SH09 only allow you to use the VCF as a processor.. also, check out this site: http://www.vintagesynth.com
*Tons* of great info, and usually they'll tell you if there's an ex/in and what part of the synth is available for processing.

 

Few synths with external audio ins have the envelope follower circuit found in stomp boxes.

That's because the envelopes are normally triggered by playing a key or an incoming MIDI note, the concept was to mix the external audio signal with the internal oscillators. So while you can process through a synth with audio inputs you will need to have a way to generate envelopes.

The audio inputs on analog synths are usually designed to handle very high voltage levels (5-15V), way more than a bass pickup puts out so a specialized preamplifier is usually required. Some modular systems (Doepfer, Blacet and synthesizers.com come to mind) offer modules that kick up the voltage and do envelope following.

As far as being a "pulse width modulator", that's nuts. PWM is applied to the oscillators of a synth, not external audio.

 

What I have done often is run my clean signal into a mixer, and one of the line-level aux sends into the modular synth. The signal gets patched into an envelope follower, a pitch-to-CV convertor, a Schmitt trigger, and an audio chain including filter/ring mod/etc.
It works best if a lot of the functions of the modular are automated by LFOs or triggered by the audio input, otherwise you have to sprout a second set of arms to control the synth while playing the bass.

I've also used the audio input on a few non-modular synths, but it's usually just a path through the filter with no dynamic control, and the signal/noise is not good.

Also, many modules are not built optimally for bass processing, so the signal kind of craps out at times, or loses low end.

Coolest effect I've gotten using a bass to control a synth? I just gated the synth output (automated or played by someone else) and side-chain triggered the gate from the bass.