Front wheel speed sensor retrofit ideas?

...but I'm not even really that sure what the hub looks like off the top of my head.

The link I posted to racerjoe's build has a picture of what he is doing. Pretty sure he cut down stock rotors for his DIY BBK, so it might give you an idea of when your hub looks. Just be aware that it looks like he used the HD rotors for his kit, and those have a cast in gusset to brace the wheel flange surface so yours might be more uniform than his is.