Airbus to Boeing headset adapter
As far as I am aware, all commercially availably XLR5 (Airbus) to Twin GA Adaptors leave pin 5 of the XLR disconnected as there nis nowhere to wire it to on the twin GA side of things.
I have checked the specs for the current HMEC 26 and Sennheiser leave the mic plug tip disconnected. This is likely to be the same on the 25.
Also bear in mind that some headsets with built in PTT's work by shorting out the tip of the mic plug to ground so there could be a conflict of interest between the headset and power bus? 
Some headsets such as the Telex 850 are powered from the mic bias voltage and these would therefore be powered through a standard Airbus to GA adaptor but not the Sennheisers.
The only way to get a panel powered Sennheiser looks to be to buy one that was manufactured with an XLR5 from the start. Cutting off the twin plugs and fitting an XLR may not work as I doubt that the wiring would be present to hook up to Pin 5 for the power.
Andy R
www.transair.co.uk