Is it permissible to log an instrument approach for purposes of IFR currency in a non-IFR certified aircraft?
As long as you remain VFR then the equipment requirements for the airplane and the flight are VFR. 

§61.57 says “Within the 6 calendar months preceding the month of the flight, that person performed and logged at least the following tasks and iterations in an airplane, powered-lift, helicopter, or airship, as appropriate, for the instrument rating privileges to be maintained in actual weather conditions, or under simulated conditions using a view-limiting device…”. 

It doesn’t say anything about having an IFR certified airplane or having a current pitot/static and altimeter check. If you are maintaining SEL IFR currency, then all you need is a SEL airplane. MEL IFR, then a multi-engine plane, etc. And if you are missing some of the 6-pack instruments, all the better for practicing partial panel.