David,
I've been performing some of my own testing using a test rig of EX4550 <--> EX2200 <-> QFX5100 all running MVRP.
Couple of really interesting points here which have snagged me up in recent testing.
1. QFX runs L2NG code is an entirely different code train (read : implementation) than EX JunOS.
2. EX needs to have the add-attribute-length-in-pdu knob enabled to be compatible with the QFX. Else, odd things can happen as the PDU is missing 2 bytes which are expected causing incorrect parsing of the PDU. Restarting l2cpd-service or deactivate/activate mvrp should clear out the cruft if it's mismatched by accident. The standard calls for the attribute-length field so the preferred scenario is to add the attribute on the EX, rather than use the remove-attribute knob on the QFX if possible.
http://www.juniper.net/documentation/en_US/junos15.1/topics/reference/configuration-statement/add-attribute-length-in-pdu-edit-protocols-mvrp.html
3. EX will announce VLANs out applicable trunk ports when EITHER an access port OR a vlan on a trunk port comes up
4. QFX will only announce VLANs ONLY when an access port comes up. In order to announce VLANs active on a trunk port, you must enable MVRP on that trunk interface (yes, even if it's facing a server/router) in addition to the standard inter-switch trunk links. The EX doesn't require this.
HTH!