Really? How? So far what I have seen is that a new block size limit in BU takes effect immediately. There is no mechanism that does the supermajority fork process.
If there is a specific option to for the supermajority fork process for a single BIP, then there should be that for every BIP. Will BU have options to allow the user to support whatever BIP or not? How will new BIPs be added? Through a software upgrade?
Yeah, software upgrade as far as I know. These are planned. Dev just started recently. Don't know how long it will take. Probably the supermajority requirements will be as in the original BIPs, with option for the user to customize them as well. Depends on what the devs do, or what people who fork BU do.*
*Note that, unlike Core or XT, it doesn't really matter (as far as consensus parameters) whether you run BU or a fork of it. This is an implication of the unbundling of consensus-parameter-setting from the rest of the code. So any questions you might be asking about the specific BU project with Andrew Stone as lead dev should probably be reconceived a bit:
Instead of asking BU what it will do, ask what anyone that does something similar could do. The genie is kind of out of the bottle. With the unbundling concept, anyone could offer blocksize-related BIP-mimicry of any kind in any configuration. It's just a matter of dev time and inclination. Dave Collins of the btcd implementation mentioned adding BU-style blocksize cap configurability, for example (on the other forum).