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The vastly better option is to obtain a prefix and ASN from ARIN and merely trade BGP with your upstream providers.
This is precisely what we are doing on the main network. We just want to keep the general browsing traffic separated.
Prefix translation comes with all the same disabilities that are present when you do this in IPv4.
In IPv4, everyone's software expects you to have a broken network (NAT) and there is lots of extra code in all of the applications to work around this.
In iPv6, it is not unlikely that this code will eventually get removed and you will then have a high level of application problems in your "prefix-translated" environment.
I am hoping that this will eventually become a moot point for this particular installation, but as it is, the cable modem is $50/month for 15 Mb/s, and adding 15 Mb/s to the main network would cost around $3,000/month. It is really hard to justify. If we could BGP via the cable modem, that would be great, but they won't allow it :) -Randy