Cisco AAA/Identity/Nac :: Implementing Mac-auth On Selected Ports Between An HP ProCurve 2510 And ACS 5.3?

I am having difficulties implementing Mac-auth on selected ports between an HP ProCurve 2510 and Cisco ACS 5.3.The 802.1x works just fine, but for selected ports I need to implement port-access with MAC-based authentication instead of regular 802.1X (yeah, I know, but this line of ProCurve switches only support one auth-mechanism per port!).The switch successfully forwards interesting MAC-auth requests for authentication to the ACS with CHAP/MD5, but the ACS reports this:

Logged At:
April 16,2012 1:20:48.080 PM
RADIUS Status:
Authentication failed : 22056 Subject not found in the applicable identity store(s). NAS Failure:
Username:
002655886b3d MAC/IP Address:
00-26-55-88-6b-3d Network Device:

[code].....

The ACS is configured to use the Internal Hosts database, where the client computer is configured like this;MAC-address: 00-26-55-88-6B-3D