I didn't look deeper into BU, but from my understanding it is not a fork like XT, but chooses the longest chain not caring about size limit. It will accept any block chain that is the longest (most hashing power). It would work with any proposed block size increase, but you still need to decide which one you support. Bitcoin XT supports BIP101. I don't know about implementations for other BIPs that also propose size increases.
If you mine for a XT pool or mine for a Core pool you are creating blocks that BU will accept. If you would like the bigger blocks to succeed you should mine at a XT pool.
Of course, you could create your own fork with and bigger block size and if you manage to have the most hashing power Bitcoin Unlimited would be ok with it.