hey m0ps, a virtual-chassis gives you all the h/a you could mostly want, there were some limitations with code upgrades on older revs, but that is roughly coming to an end. So ultimately have a multi unit chassis cluster I think will give you everything you want, just put more then 1 switch in a VC, and setup a lag between ports inside the VC.
Another option would be to use an MX which does have support for mc-lag, but the EX team didn't implement this because VCs are the way they want folks to go for inter-chassis redundancy.
It would be good for you to reach out to your SE and discuss this with them, do you know who your local sales rep and SE are?