What you want is impossible with PPP due to the encapsulation methods used to make it work. If you want VoIP to have a higher priority, then it must be set at the PPPoE client prior to encapsulation. The ePMP-SM does not do deep packet inspection and this tends to break the PPPoE encapsulation (tried to do this and found our the hardway that it breaks).
Using radius to set UL/DLMIR and some QoS settings by VSA in both the ePMP-SM and the PPPoE server works surprisingly well to do the same/similar bandwidth control. VoIP has been a challange as PPPoE makes the network look like a single hop right to the edge, not a bad thing but you loose control of your trunk (backhaul) bandwith and placing traffic controls wont work as expected as most of the data is PPP and not individually definable to http, voip, snmp, gre, etc. Using the ePMP-SM as the PPPoE client, the MIR/QoS settings work so well that we do not even set the PPPoE server VSAs in ePMP radius profiles anymore. Using any other PPPoE client, It does not matter what the PPPoE client is, if you can not set QoS in the device, then you have no control.
I am kind of curious as to why you are giving clients bridged connections? That is a L2 connection directly into your network. My position is NAT or Routed, leave bridged for PTP links. With Routed you can set priority Vlans for Voip and traffic engineering on your backhaul network provides the ability to control the network and provide high priority data the correct priority. And yes we use PPPoE on our data connections for the AAA accounting for data caps. (yes we set data caps, but are reasonable about it. Basic packages are for basic needs not for cheaply hosting torrent servers, why should the few heavy users be subsidized by the many average and light users?) Voip on a second Vlan with its own PPPoE allows us to give that Vlan a higher priority than the data vlan. Still not what you want but it does work.
* This was edited due to my not being fully on topic nor answering the question the first time around.