Sorry I hadn't managed to get on and reply sooner... but I think claytushawood hit it on the head.
Hippieguy, maybe you're not quite following what I'm saying, because it definitly doesn't "undermine the whole idea of an OBEL" -- in fact, I believe it would work exactly the same. There still could be something I'm missing (which is why I brought it up here in the first place), but I think (and I say this wishing to offer no offense) its possible you're in the mindset of "Jerry did it this way so it has to be this way."
The only reason the have an on board effects loop is so the pickup signal goes directly to the effects in order to utilize the potential of the effects reguardless of where the guitar volume is set.
To see what I am saying, change the last phrase to "regardless of where THE FINAL VOLUME is set."
So if you're in Mutron mode and you want full signal going to the effects, that's where you set the guitar's volume pot. And you still can roll back your final volume, as in a normal OBEL set up, but you now you do this using the volume
on the box.
For non-Mutron effects, setting it up as I suggested also does give you the option to turn down the pre-effects signal and still maintain your overall volume -- which could be useful at times (say, to bring an octaver divider in gradually rather than BAM!).
As I see it,
sound-wise the only difference betwen my proposal and the standard way is that the preamp comes
after the guitar volume pot (as CTW pointed out). Personally, I am skeptical that running through a passive potentiometer is going to make much difference in the final tone, although I admit don't have an experts level of understanding, and grant it could be possible.
The biggest
overall difference is in the mechanics -- reaching to a different place (eg, wherever the box is located) to adjust volume.
And I agree with those that said the simplest thing is to have it in the guitar. But because I don't have unlimited resources to throw at my musical toys, and for flexibility, I'm considering this route.
(OK, OK... I also have a tendency to want to do things a little differently.

I admit it, and it causes problems at times!

)
And I really don't need a new project, but I'll be soldering one way or the other. Jury still out for me.