The only reason to select an en-route alternate on a non-etops flight is to allow the reduction of contingency fuel below 5% of trip. You are effectively re-dispatching the flight overhead the en-route alternate or abeam it at suitable point thus enabling you to reduce the contingency to 5% of the trip from the decision point to the destination rather than 5% of the whole trip fuel. We use it quite a bit on routes that stretch a 320 or 321 to the limit such as Tehran-Heathrow, being able to lose 400kg of contingency fuel can make all the difference
As BOAC says JAR 1.297, which is available if you google it, says that at an en-route alternate has the same planning minima as a destination alternate, the rules and reference are the same under EU-Ops as far as I can tell. Quite agree that "adequate" is not simply an ETOPS term. I guess the difference with an en-route alternate is that there is a real chance of ending up there and you have to dispatch knowing that is will be available to you should you need it.