In imc-conditions the separation minimum is the prescribed radar minimum or 1000ft. MA-procedures are often made to keep you safe from terrain. Not departures ahead.
Emergency separation of 500ft is not for normal day-to-day operations. This is one of the reasons why we (where I work) increase spacing on final in LVP. Just because of the increased probability of MA. (Another reason is increased RWY occupancy time).
We also have a rule for non-lvp imc conditions that departures has to start rolling before arrival passes outer marker ~4NM to ensure separation in case of MA. I've actually never had a MA behind a departure this close in IMC, but I'm sure the required 5 NM sep minimum would be busted - so I guess this is some kind of semi-procedural control.