I know there was such a thing as a Cape Colony...I have never once denied that.
Historic borders are not a requirement for secession, but they would seriously help since your movement WILL get violent uprisings should it try force its will on the remaing 50%-1 people people, and it will get vehement opposition from the rest of the country.
As for the whole autonomous thing, the Cape is free to pass its own laws ANYWAY, within the confines of the Constitution of South Africa, and within the bounds of the national legislature, becoming any more autonomous than that is just duplicating effort for no gain. Should we become a union based on the same system as the USA, the same applies, there are federal laws that must be adhered to...
As for resistance to the idea, there basic "intention" will always get resistance, no matter how they try and present it... They're trying to break up South Africa, and "steal" one of the top 3 provincial economies, because they don't like the fact that the majority of this country have voted in the ANC. Welcome to the real world, you don't always get your own way...they need to put on a pair of big girls panties and actually do something positive for the country, and not for themselves.