The Keeley mod has two parts - the analog thing (high cut?), and the effects loop.
The analog thing would be pretty easy, in fact I've seen a tutorial somewhere written by Robert Keeley on how to do that kind of mod with any delay. I ust can't for the life of me find it again... but someone here will definately be able to point you in the right direction. Failing that, try www.experimentalistsanonymous.com/
Actually, I see you already have. Excellent.
The effects loop would be significantly trickier, you'd have to find the feedback section of the circuit and add in/out jacks so that the signal is sent through the loop before it is sent to the feedback section (I think). Theres more chance that you'll break something doing this.