Before setting a flag day, I think we should get written cooperation agreements from the largest economic players in Bitcoin. This would include:

There isn't a flag day to set. If the major economic organs like exchanges run the BIP, non-signalling miners simply wont get paid (starting October 1st) and their blocks will be rejected. Miners will have the choice to signal, or find something else profitable to mine. In turn, this will trigger the existing segwit deployment for everyone who has already upgraded to segwit compatible node software (currently Bitcoin Core 0.13.1, 0.13.2, 0.14.0, Bitcoin Knots 0.13.1+, and bcoin) regardless of whether they run this BIP or not.

But yes, it goes without saying that this BIP would need to have buy-in from major economic organs, especially fiat egress points, before being deployed. Failing that, a second deployment of segwit with a flag day, or preferably using the bip-uaversionbits-strong BIP9/flagday hybrid would be required.