I have three Deltalabs boxes of various kinds. For the centre-tapped pot on the VCO/delay mult control, I am going to use a MCP42010 dual 10k pot. A top to +5V, A bottom to ground, A wiper to pot B top, B bottom to LFO output, and B wiper to original pot wiper. I've built it but haven't written the software yet. That's a project for this coming weekend. I've been writing software for a multitap delay box I've been working on... too many projects on the go.
I haven't quite worked out the audio ones because usually you can just put in a non-centre-tapped pot, and just a 2.2k resistor from the centre tap to the wiper and it works ok. It just requires careful adjustment for the 'no feedback' position. I am looking at the AD5290 digital pot because it'll do +/- 15V, I think I might do that for the audio ones.
I looked at CT pots but the minimums were too high, as CJ and Gold both pointed out. A groupbuy might work, but my solution is pretty simple and you can get the parts from Digikey. You can even DIY a simple programmer. I should post my code somewhere once I'm finished writing it.
Not only do I fix up and use old Deltalabs boxes, but I've been on an Eventide fixer-upper kick. I got and fixed a broken H949 and now I'm waiting for some RAM for a H910. I'm finding that bringing old audio gear back from the dead is more fun than building mic capsules (at least after mic capsule #250). I'd like to fix the automation computer on my Soundcraft board as well...