Things with an @ in them are generally considered internal stuff that normal users shouldn't be using. USFM is a (very) public set of commands, the \zwhatever stuff are meant to expose complex stuff for users, and there are various other ptx2pdf macros that don't have an @ in them, those are the public interfaces. As programmers, we try to keep public-interface calling conventions unchanging so that user's modifications don't get clobbered (which can lead to VERY strange and hard to debug results). Things with an @ sign in them are assumed to be private to the macros, and so we feel free to add/remove/rearrange arguments, etc. as necessary, and as long as we cope with refactoring the code-base properly, no one should notice.
Your glossary probably starts with \mt or similar, which is a title, and so the code inserts a \mark{\t@tle} there for you. There are only 2 magic values for \mark at the moment, {\t@tle} for title pages and {} which triggers the 'no verses' header style. Otherwise the first \mark on the page gives the book-chapter-verse for the start of the page and the last one gives the end reference.
Are you using the glossary-word header? If so, then to get that for a page that starts with a title, *AND a rule*, you probably want to insert a book-hook{GLO} or a page hook to insert \mark{}.
Alternatively, if the rule isn't so important, you could use
\HEADFOOTset{H}{title}{l}{c}{r}{L}{C}{R}
to set the header contents (for a double-page spread) where l,c,r are left,centre and right contents on even numbered pages, and the capital versions are odd numbered pages.
