Neil,
The ability is there, it's not just built in yet. It's possible via RFC 4675 which was actually drafted by a couple of HP technologist. To configure it in IMC: User-->User Access Policy-->Access Device Management-->Access Device Type Select the vendor and click the configure proprietary attributes button. If it's already there, you are good, if not....
Add
Name: Egress-VLANID
ID: 56
Type: String Type
Max Length: 6
Add
Name: Egress-VLAN-Name
ID: 58
Type: String Type
Max Length: 10
One is obviously used to assign the vlan by PVID the other by name.
Then User-->User Access Policy-->Access Device Management-->Proprietary Attribute
Add a Proprietary Attribute and call it whatever you like
Select Add to Add an Attribute list and select one of the Egress-VLAN attributes you just configured (if they didn't already exist)
Check the box to send the attribute in the Access-Accept and enter the appropriate value which should be
1<VLAN-NAME (string)> The one indicates tagged use 2 for untagged (but you could just use the built-in function for untagged.
If you chose to send it by PVID I believe the format is <tagged/untagged(0x31 or 0x32)>000<VLAN_ID (as hex)> Note the 000 is a required pad and is always the same.
Once your proprietary attribute policy is configured You need to configure it as the Proprietary attribute policy in the access service being used to authenticate the client (either as the default or in the access scenario the client would match). UAM will then send the tagged vlan in the access accept and the switch should add that vlan once the client successfully authenticates.
There is also a proprietary attribute for the procurve gear (what we used before the RFC was drafted)
NAME: HP-Egress-VLANID -- ID 64 and NAME: HP-Egress-VLAN-Name -- ID:65 The rest is the same.
For more on the rfc see
http://tools.ietf.org/html/rfc4675
http://wiki.freeradius.org/vendor/HP
Hope it helps.
PL