Is the EHL required for Commercial or ag loans that may take residence as part of the collateral?

If you are talking about the EHL logo on advertisements, you would need to include it according to your regulator.

For example, the OCC does not require the logo for National Bank, though the FDIC requires this:

§ 338.3 Nondiscriminatory advertising.
(a) Any bank which directly or through third parties engages in any form of advertising of any loan for the purpose of purchasing, constructing, improving, repairing, or maintaining a dwelling or any loan secured by a dwelling shall prominently indicate in such advertisement, in a manner appropriate to the advertising medium and format utilized, that the bank makes such loans without regard to race, color, religion, national origin, sex, handicap, or familial status.

If you are talking about the EHL disclosure I have seen some banks provide with their loan documents, I believe that is a secondary market requirement as I have never been able to trace that disclosure to a compliance regulation.

Typically the Equal Housing Lender disclosure (house and "=" sign with text) is on ads for housing related loans. It's easy to do as a practice, so it isn't omitted accidentally when it is required and it hurts nothing to include it if it is not required. Even though national banks haven't been required to use this, I can't tell you the last time I saw an applicable ad without it.

Part of the KISS practice.

Is the Equal Housing Lender disclosure (doghouse) required on letters sent directly to customers about their HELOC? I don't think it's considered to be an "advertisement".

required on letters sent directly to customers

What letters?

We include the logo on any letter to a borrower when it is encouraging the borrower to contact us a finance or refinance mortgage loan (closed-end and HELOC).

The Fair Housing Act defines an advertisement as [§338.3(a)]:
A promotion for any loan for the purpose of purchasing, constructing, improving, repairing, or maintaining a dwelling or any loan secured by a dwelling.

Reg Z explains an advertisement does not include direct personal contacts …such as follow-up letters, cost estimates for individual consumers or oral or written communications relating to the negotiation of a specific transaction; Reg DD has similar wording. However, the FHA doesn't.

Thus, a conservative approach would say you should include the EHL logo in all promotions - even direct, personal contact letters. If these letters are being sent to more than one customer (commercial broadcast marketing), they definitely need it.