You really need to measure the impedance at the relative frequencies you're trying to stop getting through - and radio frequencies are a long way from a DC signal!

It's a shame that guitars aren't now shielded as standard. Most were designed in the late '40s or early '50s and the construction methods haven't really changed since then. And there is now so much electronics around us putting out noise compared to then. It really is stupid for most guitar companies to continue to think that what was OK then is still OK now.