One more point of clarification, you say "They need this to be included in the htm files that are generated when building their software".
It sounds like this element ends up in the final output, meaning that it does not necessarily need to be in the DITA content itself. I suspect what might have occurred with the older version of the DITA Open Toolkit is that it did not recognize the element as being DITA, and it had some form of bug that caused it to pass this element through to the HTML output unmodified. Either newer versions of the DITA OT have corrected this bug or they are simply validating the document properly (stopping all output).
If that is the case then replacing it with a valid DITA element and then altering the DITA Open Toolkit (its XSLT) so that it inserts <Help_anchor> into the HTML output when it sees that element (and possibly element + specific attribute value) is probably what you would want to do.
I'm not even clear on that point though, because <Help_anchor> doesn't exist as part of any HTML spec that I'm aware of, so adding it to an HTML file would make that HTML invalid as well. Perhaps the HTML is custom as well? And it is being displayed with some custom browser or other software (so not really HTML, but perhaps based on HTML)?