Uh.. the 'hack' is very simple, and isn't a hack at all, so much as it is a 'workaround' since on two different machines (an iPaq and an Axim), I've been unable to get AVRCP working out of the box without loading the Windows Media Plugin Loader.
1) Download the
WMP Plugin Host Adapter at
http://www.conduits.com/products/player/plugins.asp
2) Unzip the archive and put the GEN_WMHOST.DLL file into your PocketPlayer folder.
3) Load PocketPlayer. Make sure the BT headphone is paired to the device and there's a Wireless Stereo (MS) or High Quality Audio (Widcomm) pairing established between the PocketPC and the headphones.
4) Go into Menu -> More -> Options area. Click on "Plugins", then click on "Windows Media Host Plugin" and click 'Configure' on the context menu that follows.
5) Click "Load Plugins". When PocketPlayer asks if you want to load the "Bluetooth A2DP plugin", tell it 'yes', then say 'yes' to "Do you want to load the Play/Pause Workaround (useful for some AVRCP devices)?" button.
6) Click "Done", then close PocketPlayer out.
7) Reopen it. If you've done it all correctly, AVRCP should work fine. If it doesn't work, try reloading the Windows Media plugins again. Windows Media Player doesn't have an issue with this on either BT or Widcomm with WM5 on two different devices I've tried (hx2000 series with native Widcomm and WM5, Axim x51v with native MS and WM5).
It's odd... but it works. For some reason, I've found the Bluetooth Headphone plugin which comes with PocketPlayer 2.x and 3.0 to be particularly useless... even with a WIDCOMM device, as you discovered.