Can we make a HELOC that is secured by a second home owned by the borrower? A reference to where this topic is addressed would be appreciated.

I don't have a reference but we have several HELOCs on second homes.
We also have a few on unoccupied homes but the Note is a little different, we exclude the provision for having to occupy the property.

Commentary:

Supplement I to Part 226—Official Staff Interpretations

Section 226.5b Requirements for Home Equity Plans

1. Coverage. This section applies to all open-end credit plans secured by the consumer's dwelling, as defined in §226.2(a)(19), and is not limited to plans secured by the consumer's principal dwelling. (See the commentary to §226.3(a), which discusses whether transactions are consumer or business-purpose credit, for guidance on whether a home equity plan is subject to Regulation Z.)

Also, 226.2(a)(19) as referenced above:

2(a)(19) Dwelling .

1. Scope . A dwelling need not be the consumer's principal residence to fit the definition, and thus a vacation or second home could be a dwelling. However, for purposes of the definition of residential mortgage transaction and the right to rescind, a dwelling must be the principal residence of the consumer. (See the commentary to §§226.2(a)(24), 226.15, and 226.23.)

2. Use as a residence . Mobile homes, boats, and trailers are dwellings if they are in fact used as residences, just as are condominium and cooperative units. Recreational vehicles, campers, and the like not used as residences are not dwellings.

3. Relation to exemptions . Any transaction involving a security interest in a consumer's principal dwelling (as well as in any real property) remains subject to the regulation despite the general exemption in §226.3(b) for credit extensions over $25,000.