I meant to follow up on my attempts to stack with 10GBASE-T transceivers a while back but I've been busy.I've still been playing around and trying to get stacking working with 10GBASE-T transceivers between two 7250-48p switches. Nothing I've tried seems to work. ...
To anyone else thinking of stacking with 10GBASE-T transceivers: It doesn't appear to be possible on the ICX 7250 switches (and likely all other ICX switches as well). I imagine the desire to stack over CAT6 is rather limited, but hopefully this saves someone a headache.
It isn't a design limitation with the switches, it's an issue with the transceivers. The 10GBASE-T transceivers can only send/receive ethernet data/frames (IEEE 802.3) whereas fiber transceivers and/or DACs are raw data connections that can be used for other, non-ethernet connections (Fiber Channel, for example). As far as I can tell, this is true of every current 10GBASE-T transceiver on the market (understandably; they're RJ45 adapters, after all).
The ICX switches appear to negotiate via ethernet and then switch to communicating between stack members via a raw connection. This is why the interfaces appear to flap when attempting to stack with 10GBASE-T transceivers. The switches bring the stacking interfaces up, figure out who's on the other end, and then switch to a raw data connection. Since the transceivers don't support non-802.3 operation, they promptly disconnect. The switch the resets the interfaces and the cycle repeats.
One of my searches while trying to figure this out rendered this internal Brocade slide-deck on stacking: stack-unit - Brocade Community Forums (edit: Despite the title, this isn't a link to the Brocade forums; it's to "studylib.net") -- Notable slides that support this theory are 30 & 31.