Interesting request.
I think you will be able to generate single Client Certificates (so just each one Client Certificate could be used with each one IP Phone) and not just only one for all of your IP Devices.
HiPath DLS (now OpenScape DLS), which acts as a central configuration and deployment point of security parameters (I mean VoIP Security Management and/or 802.1x Certificates Management), should then let you manage the 802.1x authentication process through the definition and usage of Templates/Locations profiles by using IP Devices or SIP Users specific information/parameters so you can manage what you want per-Device basis (An example: I think a parameter for a custom Location could be an IP range and then some IP Devices could be associated with that custom Location...but other parameters are available, like E.164 Patterns).
As references I would read the "OpenScape Deployment Service Administration and Installation Manual" (latest available edition: P31003-S2370-M107-01-76A9 released September, 13rd 2013) and the "IEEE 802.1x Configuration Management Administration Manual" (latest available edition: P31003-S2000-M103-01-76A9 released April, 22th 2013) and I would use the OpenScape DLS V7 R1.10.2 (recently released) or the OpenScape DLS V6 R1.7.1 (better R1.8.0 recently released) taking care of reading relevant Release Notes too (The reference about those specific DLS versions is due to resolved issues with Plug & Play Deployment of PKI Certificates and Automatic Certificates Renewal features).
Also consider to search (SEBA now Unify Partner Portal) Knowledge Base for articles about IEEE 802.1x: there you will find interesting things. Personally I've not enough experience about IEEE 802.1x deployments because (as far as I know) such type of security requirements isn't so common here (gosh!) but, maybe, other Forum's members were able to help you better than me.
Best regards, Kimera.