25% of WD40 is oil, so if you don't mind adding more oil to the strings...
I actually found an unused Tibetan almond stick that I've tried for cleaning strings, just for fun. Seems to work fine. Another option is getting FingerEase, spray a little bit on a micro fiber cloth and wipe the strings with this.
Personally I wouldn't feel comfortable doing that. Aside from WD-40's ability to attract dirt in electronics, the stuff doesn't seem to evaporate easily. I would be very nervous what it might due to wood.
Good luck, hope its good, I'll stick with finger ease and wonder wipes
Cheaper than dedicated products yes, but I'm with mikeller on this, I'd be worried about the attraction... fast fret and string changes every so often for me......