Thanks for all your education, everyone.
It seesm to me then, that, for the Seneca:
1. A 150' EOA should be applied to the 'symmetric' DA/DH.
2. A VCH of 350' should be used; at 350' a.g.l the IAS and configuration should be progressively adjusted to achieve Vref and full flap at the threshold.
Thus, if landing criteria have not been achieved on an asymmetric ILS (with a 'normal' DH of 200 ft), an asymmetric DH of 350 ft must be used and a go-around initiated if landing criteria have not been achieved at 350 ft.
No attempt at an asymmetric go-around should be flown below 350 ft.